Conchological and molecular analysis of the “non-scaly” Bornean Georissa with descriptions of three new species (Gastropoda, Neritimorpha, Hydrocenidae)

Abstract The Bornean representatives of the genus Georissa (Hydrocenidae) have small, dextral, conical, calcareous shells consisting of ca. three teleoconch whorls. Our recent study on the Georissa of Malaysian Borneo has revealed high intra- and inter-specific variation in the “scaly” group (a group of species with striking scale-like surface sculpture). The present study on the “non-scaly” Georissa is the continuation of the species revision for the genus. The “non-scaly” species are also diverse in shell sculptures. This informal group comprises Georissa with subtle spiral and/or radial sculpture. The combination of detailed conchological assessment and molecular analyses provides clear distinctions for each of the species. Conchological, molecular, and biogeographic details are presented for 16 species of “non-scaly” Georissa. Three of these are new to science, namely Georissacorrugatasp. n., Georissainsulaesp. n., and Georissatrusmadisp. n.


Introduction
The genus Georissa Blanford, 1864 (Hydrocenidae) comprises minute terrestrial snails, generally characterized by a small, dextral, conical, calcareous shell with ca. three teleoconch whorls (Bandel 2008; Thompson and Dance 1983;Vermeulen et al. 2015). Georissa is primarily restricted to environments rich in calcium carbonate (CaCO 3 ). They are found in variable abundances on wet and shaded limestone walls or rocks, but occasionally on sandstone rocks and in vegetation that is not associated with a rocky substrate (Haase and Schilthuizen 2007;Khalik et al. 2018). They have a calcareous operculum, constructed in a concentric paucispiral manner and a peg attached at the inner surface. The hemi-spherically shaped protoconch has a distinct microsculpture, which often shows species-specific distinctness (Khalik et al. 2018).
Until recently, simple conchological analyses have been the main approach to describe and study the species of Bornean Georissa. In our recent systematic study of the "scaly" Georissa (see Khalik et al. 2018), however, we combined molecular, detailed conchological examination and biogeographic data of each species to assist in the process of species delimitation. We have revealed that this group of minute land snails has high intra-and inter-specific variation, especially in shell shape, size, aperture, and sculptural characters, as well as high allopatric diversity. Here, we provide a complete list of known "scaly" Georissa of Borneo: G. scalinella (van Benthem-Jutting, 1966); G. saulae (van Benthem-Jutting, 1966); G. hosei Godwin-Austen, 1889; G. anyiensis Khalik et al., 2018; G. muluensis Khalik et al., 2018; G. hadra Thompson & Dance, 1983; G. kobelti Gredler, 1902; G. niahensis Godwin-Austen, 1889; G. silaburensis Khalik et al., 2018; G. bauensis Khalik et al., 2018; G. pyrrhoderma Thompson & Dance, 1983; G. kinabatanganensis Khalik et al., 2018; G. sepulutensis Khalik et al., 2018. Striking allopatric patterns are well-known from several other microsnail taxa of Southeast Asia (see Liew et al. 2014, Hoekstra and Schilthuizen 2011, Rundell 2008, Tongkerd et al. 2004). These studies have led to the realisation that the geographic variation of different populations needs to be well understood and used as an important guideline for species delimitation. Previous phylogenetic studies on the Bornean Georissa based on 16S and CO1 mtDNA allowed species to be recognised as monophyletic clades. There were at least two exceptions to this pattern, G. kobelti and G. saulae, which are paraphyletic with respect to the locally endemic, conchologically distinct G. niahensis and G. filiasaulae, respectively (Khalik et al. 2018;Schilthuizen et al. 2005). Such paraphyletic patterns are not unexpected when microgeographic speciation yields recently evolved; locally endemic species branched off from more widespread ancestors (Schilthuizen and Gittenberger 1996).
In this paper, the second part of our work on the Bornean Georissa, we apply the same approach of combining information from multiple datasets to 16 species of Bornean Georissa that belong to the informal "non-scaly" group, characterised mainly by weak to strong spiral and/or radial sculptures without conspicuous scale-like sculpture on the shell. We also present the phylogenetic relationships among all Bornean Georissa and their distribution. We describe three species new to science, namely Georissa corrugata sp. n., Georissa insulae sp. n., and Georissa trusmadi sp. n.

Micro-computed tomography (µ-CT).
The µ-CT scanning was carried out using an Xradia 520 Versa X-ray Microscope (see Suppl. material 1 for µ-CT scanning parameters). We obtained ca. 995 layers of X-ray images of an individual shell per scanning, which we then used to reconstruct a composite 3-dimensional image of the shell. These images were then segmented in Avizo ver. 9.4.0 (FEI Company), to examine the operculum, peg, and inner part of the shell.

Molecular analysis
DNA extraction. The sample preparation prior to DNA extraction procedure followed the method from Khalik et al. (2018). We extracted the genomic DNA from 52 individuals using the Qiagen DNeasy Blood and Tissue kit, and applied the protocol provided by the manufacturer.
DNA Sequencing. PCR products were sent to BaseClear B.V. (Leiden, The Netherlands) and Sanger sequenced in forward and reverse directions using the ABI3730XL sequencer, Life Technologies.

Sequence alignment and phylogenetic analyses
Sequence data. From GenBank, we downloaded 16S and CO1 mtDNA sequences of representatives of the "scaly" group species, G. gomantonensis (Khalik et al. 2018), a full mitochondrial genome of G. similis (Uribe et al. 2016), and, as an outgroup, Bathynerita naticoidea (Arellano et al. 2014). We extracted the 16S and CO1 regions from the G. similis full mitochondrial genome to be included among the sequences in our phylogenetic analysis. The newly sequenced data were assembled using de novo Geneious 10.2.3 assembler, manually edited, and trimmed for ambiguities. This resulted in a total of 68 and 55 sequences of 16S and CO1 mtDNA, respectively. Sequences were deposited in GenBank via BankIt (https://www.ncbi.nlm.nih.gov/WebSub/) and BOLD (http://boldsystems.org/). Sequence alignment. The 16S and CO1 mtDNA sequences were aligned to their respective genes using default parameters of MUSCLE (Edgar 2004). The alignments were manually checked and edited.
Phylogenetic inference. The alignment of CO1 mtDNA was set to invertebrate mitochondrial genetic code at the third reading frame. The best fit nucleotide substitution models of the concatenated 16S and CO1 sequence alignment was determined using ModelFinder (Kalyaanamoorthy et al. 2017) based on corrected Akaike Information Criterion (AICc). The best-fit nucleotide model for the concatenated sequence alignment is GTR+F+R4. Phylogenetic analysis. We performed a maximum likelihood analysis using the concatenated alignment using GTR+F+R4 nucleotide substitution model with ultrafast bootstrapping (5000 replicates) (Hoang et al. 2017) in IQ-TREE 1.6.3 (Nguyen et al. 2015). We used MrBayes 3.2.6 (Huelsenbeck and Ronquist 2001) for Bayesian Inference using the following settings: GTR+I+G nucleotide substitution model; 1,100,000 number of generations; tree subsampling for every 200 generation; 100,000 burn-in length; 4 heated chains with heated chain temperature at 0.2. Details of the newly sequenced data and their accession number are listed in Table 1.

Species delimitation and description
Species delimitation of the "non-scaly" group Bornean Georissa was carried out based on detailed examination of the shell characters which are exclusive to the group, combined with the molecular analyses. While morphological analysis is widely accepted for species identification in gastropods, this conventional way of species delimitation could become very challenging when applied to the genus Georissa which show high morphological variation within and between populations. For this reason, we applied a similar species delimitation approach as done in the "scaly" group Georissa (Khalik et al. 2018). In view of the considerations given in Khalik et al. (2018), we refrained from web-based species delimitation in this case.

CO1 genetic divergence
CO1 genetic divergence was performed to determine the genetic distances between species of the "non-scaly" group Georissa. We conducted genetic distance analysis within and between species groups. We computed pairwise genetic distances of CO1 sequence alignment based on the nucleotide substitution model Kimura 2-parameter in MEGA v. 7.0.26 (Kumar et al. 2016) which includes the transition + transversion, gamma distribution, and 1000 bootstraps for variance estimate. We conducted the analysis based on CO1 sequence data of 40 individuals comprised of nine species, including three newly described species.

Morphological and phylogenetic analyses
The "non-scaly" Georissa from Borneo are characterised by the simple spiral and/or radial sculpture on the shell, unlike the distinct scale-like structures of the "scaly" group.
These two informal groups of Georissa could be used as an initial framework for future species identification. Previously, Thompson and Dance (1983) divided the Bornean Georissa into four groups, namely the "hosei", "borneensis", "everetti", and "williamsi" groups. The "hosei" group and a species of the "borneensis" group (i.e., G. pyrrhoderma) are species with scaly sculpture. Thompson and Dance (1983) included G. monterosatiana from Peninsular Malaysia in the "hosei" group, which does not have obvious scales on the shell. The rest of the groups of Thompson and Dance (1983) consist of the "non-scaly" species, which were further distinguished based on their colour and ribbing. Although shell colour may help in species-level taxonomy, we suggest not to use colour as a character for species grouping, given the wide range of shell colour variation in most Bornean Georissa.
Our previous work on the "scaly" group Bornean Georissa (Khalik et al. 2018) together with this present study on the "non-scaly" group have resulted in a complete revision of the Bornean Georissa. To date, we recognise 29 species of Bornean Georissa, of which 13 are in the "scaly" group and 16 are in the "non-scaly" group. Since we have studied and examined all shell materials from BORN, MZU, ZMA, RMNH, MFN, NHMUK, and JJV, we find that it is useful to highlight some issues related to the "non-scaly" group that could be beneficial for future understanding. Firstly, the name G. williamsi was mentioned in several publications to refer to a species with distinct spiral ribs (Thompson and Dance 1983;Clements et al. 2008;Nurinsiyah et al. 2016;Maassen 2003;O'Loughlin and Green 2016;Vermeulen and Whitten 1998). After examination of the holotype of G. williamsi in the NHMUK, we find that this species name has often been misapplied. The images provided by Thompson and Dance (1983, figs 66-68), Phung et al. (2017, fig . 8C), and Vermeulen and Whitten (1998, fig. 15) show entirely different spiral sculpture than the 'true' G. williamsi. Based on the taxonomy presented in this paper, the specimens illustrated in Thompson and Dance (1983) are G. bangueyensis Smith, 1895;those in Phung et al. (2017) are G. insulae sp. n.; and that in Vermeulen and Whitten (1998) There is a similar confusion with G. borneensis, a name widely applied to both G. similis and G. corrugata sp. n. in the collection materials. Schilthuizen et al. (2003) mentioned G. similis, but it is presently not sure if this refers to the true G. similis or otherwise, because the collection numbers of the specimens used in their studies (materials deposited in BORN/RMNH) was not mentioned. Georissa similis and G. corrugata are conchologically distinct from G. borneensis (see detailed description in Systematic part). Clements et al. (2008) and Schilthuizen et al. (2003Schilthuizen et al. ( , 2011 refer to several species of "non-scaly" Georissa, namely, G. borneensis, G. bangueyensis, G. similis, and G. williamsi. Again, we cannot be sure whether the specimens were correctly assigned since we could not examine the materials studied by these authors, due to similar case as above.
On the one hand, we find that "non-scaly" Georissa have strongly supported monophyletic groups with bootstrap and posterior output values in our phylogenetic analyses, ranging from 96-100 and 100, respectively. This corresponds to conchological characters of the respective taxa. On the other hand, we find G. xesta is paraphyletic. Discussions for each species treatment are in the Systematic part.

CO1 genetic divergence
Species delimitation based solely on morphological analysis of this group of closely related minute gastropods could be challenging, especially when the studied taxa have high intraspecific variation (see Khalik et al. 2018;Liew et al. 2014). The analysis of molecular data provides a large benefit in the process of species delimitation. Previous systematic studies of gastropods have reported to successfully delimit the studied taxa to a species level by using CO1 divergence (see Boeters and Knebelsberger 2012;Liew et al. 2014, Khalik et al. 2018Puillandre et al. 2012), but provide no specific genetic barriers for each studied taxon. The CO1 genetic divergence (Table 2) shows the Kimura 2-parameter distances within a group of species and net average distances between groups of Georissa sequences. This reveals that between-species genetic divergence of the "non-scaly" species exceeded 0.10, with the exception of G. xesta vs. G. flavescens, G. xesta vs. G. bangueyensis, and G. xesta vs. G. nephrostoma. Although the divergences of these species pairs are considerably low, they comprise groups of species with distinct morphological characters. This is similar to what was found with the "scaly" Georissa, for example, G. silaburensis vs. G. bauensis is a conchologically distinct species pair that has a CO1 divergence as low as 0.04 (Khalik et al. 2018). We also find that the intraspecific divergence within each "non-scaly" species is equal or does not exceed 0.05, with the exception of G. xesta (0.11). Hoekstra and Schilthuizen (2011) suggested that intraspecific divergence for a limestone-dwelling microsnail (Gyliotrachela hungerfordiana Möllendorff, 1886) of Peninsular Malaysia would not exceed 0.10, which we find in the Bornean Georissa as well, with the exception of G. xesta. We find that the genetic divergence analysis of the Bornean Georissa provides useful information for species delimitation. There is, however, no specific genetic divergence limit that separates intraspecific from interspecific distances, since the divergence within a species and divergence between species often overlap (the highest value for intraspecific divergence = 0.11, while the lowest value for intraspecific divergence = 0.03).

Systematic part
Class Gastropoda Cuvier, 1797 Family Hydrocenidae Troschel, 1856 Genus Georissa Blanford, 1864 "Non-scaly" group We previously described the first informal group of Bornean Georissa, the "scaly" group which consists of 13 species (Khalik et al. 2018). In the current paper, we describe the remaining group of Bornean Georissa, consisting of 16 species which do not have conspicuous scale sculpture and are characterised mainly based on species-specific patterns of more subtle radial and/or spiral sculpture. Our "non-scaly" group corresponds to Thompson and Dance's (1983) "williamsi", "everetti" and "borneensis" (p.p.) groups. A species of the "borneensis group", Georissa pyrrhoderma Thompson & Dance, 1983 has been previously included by us in the "scaly" group (Khalik et al. 2018).  Phylogenetic analyses were conducted using concatenated sequence alignments of partial 16S and CO1 mtDNA. The analyses consist of 69 ingroup taxa (11 taxa representing the "scaly" group and 58 taxa representing the "non-scaly" group), and Bathynerita naticoidea as an outgroup. "Scaly" taxa in the phylogenies are with the red branches. General conchological description of a "non-scaly" group representative. Protoconch. Colour (in living or freshly dead specimens): white, yellowish green, orange, red, or brown. Sculpture pattern: smooth (no sculpture on the protoconch), straight lines (the sculpture is raised in a pattern of straight lines), rounded to ellipsoidal (the sculpture is rounded and/or ellipsoidal), mixed (a combination of more than one sculpture patterns), or irregular (the present sculpture comprises of no uniform shape or pattern). Teleoconch. Colour (in living or freshly dead specimens): white, yellowish green, orange, red, or brown. First and subsequent whorls: convex (the whorls are partially circular in shape), rounded (the whorls are semi-circular in shape), and/or flat. Suture: deeply impressed. Shoulder: narrow or extended. Number of whorls: 2 ¼-3 ½. Shell height (SH): 0.62-2.23 mm. Shell width (SW): 0.60-1.82 mm. Shell index (SI=SH/SW): 0.97-1.51. Shell sculpture. Radial sculpture: absent or present; if present then either raised in wavy and/or regular form, with narrow or wide interval. Growth lines: weak or strong, for species without clear formation of radial sculpture. Species with radial sculpture normally do not have clear growth lines since these are covered by the radial sculpture; such species generally have a row of nodules at the shoulder close to and parallel to the suture or away from the suture on the whorls. Spiral sculpture: absent, weak or strong, continuous or discontinuous, frequently the orientation is distorted by the radial sculpture (if present). Columella. Smooth and translucent. Umbilicus: open or closed. Aperture. Shape: semi-elliptic, ovoid or rounded, with straight, concave or convex parietal side, palatal edge either contiguous with the body whorl or with the parietal side. Aperture height (AH): 0.31-1.07 mm. Aperture width (AW): 0.33-1.09 mm. Aperture index (AI = AH/AW): 0.81-1.02. Peristome. Simple, thickened inside, sharp toward the edge of the aperture. Operculum. Shape: ovoid to rounded, the inner surface of the operculum has a small crater-like structure next to the peg. Peg: straight or curved. The shell dimensions of the "non-scaly" Georissa are summarised in Suppl. material 2.
All species of Bornean Georissa have a broadly developed callus that fully covers the umbilicus, except G. leucococca, which has this callus incompletely developed. Hence, the umbilical region of this species is partially open. Georissa nephrostoma is the only known Bornean Georissa with a 'bulb'-like callus covering the umbilical region. This is an inflation of the columella along the parietal wall. As a result, the aperture of G. nephrostoma is partly obstructed, unlike any other aperture of the Bornean Georissa. Of all the "non-scaly" Georissa, the operculum is available, except for G. corrugata, G. williamsi, and G. leucococca.
Habitat and ecology. Like the "scaly" group, the members of the "non-scaly" group Georissa are usually restricted to limestone areas. They can be found on the limestone walls, rocks located in wet and shaded environments, and occasionally at a low density on dry limestone walls and rocks, in the vegetation away from the limestone (e.g., G. gomantonensis), on other, non-limestone rocky substrates (e.g., G. saulae), and on limestone walls inside cave systems with partial or no exposure to the sunlight (e.g., G. silaburensis and G. filiasaulae).
Distribution. We provide distribution maps of the "non-scaly" Georissa of Malaysian Borneo in Figures 3, 4. The species are divided into two distribution maps to avoid  overlapping. There are at least twelve species of the "non-scaly" group in Sabah, two species in Sarawak, and another two species in both Sabah and Sarawak.
Remark. For the type material that was not examined during this study, there is a note in each of the species treatment that the type specimen was not seen.
In the following systematic descriptions of the "non-scaly" Georissa, the species treatment is arranged partly based on the molecular phylogeny ( Fig. 2A, B). We start with the description of six species for which no DNA-data are available, namely (i) Georissa borneensis Smith, 1893, (ii) Georissa corrugata sp. n., (iii) Georissa everetti Smith, 1895, (iv) Georissa williamsi Godwin-Austen, 1889, (v) Georissa trusmadi sp. n., and (vi) Georissa leucococca Vermeulen, Liew & Schilthuizen, 2015, followed by the remaining ten species, treated in the order in which they appear in the phylogenetic tree. The numbers of individuals of the newly described species are stated in brackets (if available) right after the collection number. The locality data may contain the following Malay words: Batu = rock; Bukit = hill; Gua = cave; Sungai/Sungei/Sg. = river; Gunung/Gunong = mountain; Pulau = island; Kampung = village. Description. Protoconch. Colour: white to pale orange, darker than the rest of the shell. Sculpture pattern: irregular sculpture pattern, from base to apex end with no specific sculptural shape. Mesh width: 2.5-8.0 µm. Teleoconch. Colour: white to pale orange. First whorl: flat, convex close to the suture. Subsequent whorls: flat, convex and angular at the periphery. Suture: clearly impressed. Shoulder: narrow. Number of whorls: 2 ¾-3 ½. SH: 1.91-2.23 mm. SW: 1.65-1.82 mm. SI: 1.12-1.28. Shell sculpture. Radial sculpture: present, weak or flattened, densely sculpted on the whorls, ca. two to three ribs per 0.1 mm. Spiral sculpture: present, but thin and weak, only visible under high magnification (> ×100 magnification), strongest at the first whorl, weaker at subsequent whorls. Aperture. Shape: semi-elliptic, straight parietal side, palatal edge contiguous with the body whorl, palatal side tilted and angular, basal side convex. AH: 0.82-1.07 mm. AW: 1.00-1.09 mm. AI: 0.75-1.02. Diagnosis. The flat whorls that are strongly convex at the periphery, giving the shell an angular shape, are diagnostic. The sculpture of G. borneensis resembles that of G. similis and G. corrugata, but is weaker and more flattened than in those species. The spiral sculpture of G. corrugata is also more irregular. The adult shell G. borneensis is larger than in adult G. similis and G. corrugata. Additonally, the base to apex end sculpture of the protoconch of G. borneensis is distinct compared to these species (G. similis has a rounded protoconch sculpture and G. corrugata has straight-line protoconch sculpture).

Georissa borneensis
Distribution. The species is known only from Gomantong hill in the Kinabatangan region of Sabah.
Discussion. The identification of G. borneensis can be confusing when we refer to the sketches by Thompson and Dance (1983: figs 61, 62). These appear to reflect the radial sculpture of G. corrugata, which is wavy/irregular and strongly sculpted. Smith (1895) and Thompson and Dance (1983) described G. borneensis referring to the absence of spiral sculpture, which, however, is present but only visible under high magnification. The bright red colour of the peristome as described by Smith (1895) could not be observed by us, probably due to the faded condition of the shells. We also find there is no association based on colouration of G. borneensis with G. pyrrhoderma and the "williamsi". Thompson and Dance (1983) grouped the 'borneensis' based on their reddish shell colour, while 'williamsi' with their light brown colour, of which we find these colours are often a variation within these groups of species. Etymology. The name is derived from a Latin word corrugatus, meaning "wrinkled", referring to the coarse and irregular radial sculpture.
Diagnosis. The wavy and irregular, widely spaced and strong radial sculpture, with thin regularly arranged spiral sculpture in between is diagnostic for G. corrugata. Georissa similis and G. borneensis have a somewhat similar arrangement of radial and spiral sculpture, but do not carry the protoconch sculpture consisting of parallel lines. The shell shape and size of G. corrugata are similar to G. similis. Besides the difference in the protoconch sculpture, the latter species also has more densely arranged radial sculpture on the teleoconch.
Distribution. Georissa corrugata is distributed on the limestone hills of the lower Kinabatangan valley, known to occur from Gomantong to Batu Tomanggong, but always in low densities compared to other Georissa species. The species has also been found in the Sepulut valley, ca. a hundred km further to the southwest. Description. Protoconch. Colour: orange to red. Sculpture: rounded to ellipsoidal mesh pattern, mixed with irregular sculptural shapes (whenever two or more rounded or ellipsoidal meshes are connected or combined). Mesh width: 4-30 µm. Teleoconch. Colour: orange to red. First whorl: convex. Subsequent whorls: convex, with relatively wide penultimate and final whorls. Suture: clearly impressed. Shoulder: narrow. Number of whorls: 2 ¾-3 ¼. SH: 1.82-2.23 mm. SW: 1.52-1.75 mm. SI: 1.16-1.30. Shell sculpture. Radial sculpture: present, thin, forming small nodules when intersecting with spiral sculpture; these nodules are also present on the shoulder close to the suture. Spiral sculpture: present, thin, regularly spaced, oblique, appearing immediately after the protoconch, distorted/discontinuous by radial ribs. Aperture. Shape: rounded to ovoid, straight to concave parietal side, palatal edge contiguous with the body whorl, basal side convex. AH: 1.05-0.92 mm. AW: 1.09-0.96 mm. AI: 0.89-0.99.

Georissa everetti
Diagnosis. The strong and thin oblique spiral sculpture on its shell is diagnostic for G. everetti. Georissa similis has a somewhat similar knitted sculpture pattern resulting from the intersection of radial and spiral ribbing, but the shell shape is entirely distinct, with broad penultimate and final whorls. Based on the shell shape and habitus, G. everetti resembles G. gomantonensis and G. williamsi, which, however, have clear, regular, spiral shell ribs. Distribution. Georissa everetti is widely distributed in Sabah and Sarawak, but is found in low abundances. The species known to occur from Padawan/Penrissen, Sarawak in the South (where Rumbang, the type locality is located), to further north, Niah, Sarawak, and Sepulut valley, Sabah. Description. Protoconch. Colour: orange to red. Sculpture: a mix of rounded, ellipsoidal to irregular sculptural shape. Mesh width: 2-6 µm. Teleoconch. Colour: orange to red. First whorl: convex. Subsequent whorls: convex. Suture: clearly impressed. Shoulder: narrow. Number of whorls: 3-3 ¼. SH: 1.58-1.91 mm. SW: 1.17-1.42 mm. SI: 1.30-1.38. Shell sculpture. Radial sculpture: absent, only weak to strong growth lines are visible at irregular intervals. Spiral sculpture: present, thin, regularly spaced at the first whorl, appearing immediately after the protoconch, on later whorls the spiral sculpture weakens and becomes distorted by the growth lines; more than 20 spiral ribs on the body whorl of the adult individual. Aperture. Shape: semi-elliptic, straight to concave parietal side, palatal side rounded, palatal edge contiguous with the body whorl, basal side slightly convex. AH: 0.63-0.81 mm. AW: 0.71-0.87 mm. AI: 0.89-0.95.
Diagnosis. Georissa williamsi has a broad final whorl, in which it is similar to G. gomantonensis and G. everetti. However, these three species are all distinctly sculptured, where G. gomantonensis has raised spiral sculpture, G. everetti has oblique spiral sculp- ture, but G. williamsi has thin, hardly raised, and densely arranged spiral sculpture (4-6 ribs in every 0.1 mm), despite the similar shell habitus.
Distribution. Georissa williamsi occurs over a large part of Sabah from the Sepulut valley in the west-central to Tawau and Lahad Datu in the east.
Discussion. The type locality of G. williamsi is 'Borneo', with no specific location stated by Godwin-Austen (1889). Saul (1966) in her note on "Shell collecting in the limestone cave of Borneo" mentioned that during her trip to Lahad Datu, Sabah, they collected G. williamsi (syn. Hydrocena williamsi). Based on the characters of G. williamsi described by Godwin-Austen (1889) and the type material we have examined, the species does not have very prominent spiral sculpture. Georissa williamsi was previously misinterpreted as having highly raised spiral sculpture, and the name was therefore misapplied to forms like G. hungerfordi, G. insulae, and G. javana (Thompson and Dance 1983;Phung et al. 2017;Vermeulen and Whitten 1998). Etymology. The species is named after the type locality, Gunung Trus Madi, Sabah. Description. Protoconch. Colour: orange. Sculpture pattern: rounded to irregular sculptural shape. Mesh width: 3-30 µm. Teleoconch. Colour: orange. First whorl: convex. Subsequent whorls: convex. Suture: well-impressed. Shoulder: narrow. Number of whorls: 2 ¾-3 ½. SH: 1.40-1.89 mm. SW: 1.12-1.37 mm. SI: 1.22-1.38. Shell sculpture. Radial sculpture: absent, weak growth lines present throughout the shell surface. Spiral sculpture: present, strong spiral ribs, broadly spaced above the whorls, ca. 5-7 strongly raised spiral ribs on the body whorl of the adult individual, appearing immediately after the protoconch, thin spiral ribs in between the stronger ones, more densely spaced and weaker at the basal part of the body whorl. Aperture. Shape: rounded to slightly ovoid, straight to convex parietal side, palatal edge contiguous with the parietal side, basal side convex. AH: 0.59-0.72 mm. AW: 0.66-0.79 mm. AI: 0.85-0.91. Holotype dimensions. SH: 1.67 mm, SW: 1.28 mm, AH: 0.68 mm, AW: 0.75 mm.
Diagnosis. Georissa trusmadi is characterised by the highly raised spiral sculpture. The number of strong spiral ribs on the first whorl is lower (3-5) than on the later whorls (5-7). The spiral sculpture is similar to G. insulae and G. hungerfordi, but always shows fewer ribs. Based on the shell habitus, it is similar to G. hungerfordi from Sarawak. The latter species, however, has the spiral ribs on the body whorl less strongly raised.
Distribution. Georissa trusmadi is only known from Gunung Trus Madi, Sabah.    Diagnosis. Georissa leucococca has spiral sculpture that is more prominent at the upper part of the whorls, similar to G. bangueyensis. Georissa leucococca is so far the only known Bornean Georissa with an open umbilicus and with an adult shell size of hardly more than 1 mm. It has an angular shell shape, similar to G. borneensis.
Diagnosis. Georissa hungerfordi is characterised by the strong spiral sculpture with ca. 7-10 spiral ribs on the body whorl. The shell shape approaches the shape of G. trusmadi and G. pachysoma. Georissa hungerfordi has stronger spiral sculpture compared to G. pachysoma but weaker compared with G. trusmadi. The spiral ribbings resemble G. insulae, which has, however, a more elongated and slender shell shape.
Discussion. Thompson and Dance (1983) considered G. hungerfordi a junior synonym of G. williamsi. We are, however, of the opinion that G. hungerfordi is a valid species based on the distinctly raised spiral sculpture of the shell compared to G. williamsi. Georissa lowi (Smith, 1893) is a junior synonym of G. hungerfordi. See also discussion in G. williamsi Godwin-Austen, 1889.
Molecular analysis. ML and Bayesian analyses show that the samples of G. hungerfordi (16S: n = 14; CO1: n = 11) form a monophyletic group with 100% BS and 100% PP, sister group to the "scaly" group, except G. saulae.
Diagnosis. Georissa gomantonensis is characterised by its bright greenish yellow colour, broad final whorl, and strong spiral sculpture. The spiral sculpture pattern is similar to G. insulae, but the shell habitus of G. gomantonensis is much broader and inflated. Georissa everetti and G. williamsi have a similar shell habitus as G. gomantonensis, but differ because lacking of the oblique (G. everetti) and densely arranged (G. williamsi) spiral sculpture.
Distribution. Georissa gomantonensis is widely distributed throughout Sabah. More commonly found in the vegetation of the limestone forest, rather than on the limestone rocks themselves. Molecular analysis. ML and Bayesian analyses show that the individuals of G. gomantonensis (16S: n = 2; CO1: n = 2) form a monophyletic group with 100% BS and 100% PP, sister group to the paraphyletic G. saulae + G. filiasaulae.  (Fig. 13).
Diagnosis. Georissa filiasaulae has weak, thin, and densely arranged spiral sculpture with nodular structure on the shoulder. The shell colour and thickness are most similar to G. corrugata, which has white and partially transparent shell.
Distribution. Georissa filiasaulae is a cave specialist, known from the cave system of Batu Sanaron and Batu Tinahas in the Sepulut valley. Schilthuizen et al. (2012) studied the population genetics of G. filiasaulae and its sister species, G. saulae. They found narrow hybrid zones between the two species in cave entrances. Molecular analysis. ML and Bayesian analyses of G. filiasaulae (16S: n = 3; CO1: n = 3) show that G. filiasaulae form one clade with 98% BS and 100% PP. The sister group is the G. saulae population from Pungiton (G. saulae is paraphyletic).
Discussion. Georissa filiasaulae is one of the two known Bornean Georissa that is troglobitic. Khalik et al. (2018) described G. silaburensis, another species of Bornean Georissa from the "scaly" group as a possible troglobite from Gunung Silabur, Serian, Sarawak. Georissa filiasaulae differs from G. saulae by the absence of any scale-like sculpture, reduced shell pigmentation, and relatively larger shell size and broader shell shape. Population genetic studies suggest that the hybrid zone between the two is restricted to a narrow region at the cave entrances, rendering the two species as independent evolutionary units. Therefore, considering them as separate species is warranted (Schilthuizen 2000).
Diagnosis. Georissa insulae is characterised by the strong and regularly spaced spiral ribs throughout the entire shell. This shell sculpture is similar to that of G. hungerfordi and G. trusmadi, but less raised than in these two species. Georissa insulae has a greater number of spiral ribs, ca. 10-12 ribs on its shell compared to these two species. The shell habitus is distinctly elongated compared to other spirally ribbed Bornean Georissa.

Georissa pachysoma
Diagnosis. Georissa pachysoma is characterised by a dark red to brown colour of the shell, similar to G. nephrostoma, but the latter species has wavy spiral ribs while G. pachysoma does not. In shell habitus, G. pachysoma closely resembles G. hungerfordi, but the colour and spiral sculpture of G. hungerfordi (orange in colour in living or freshly dead specimens, with highly raised spiral sculpture) is entirely different from G. pachysoma.
Diagnosis. Georissa similis is characterised by the dense radial sculpture. The radial ribs intersect with the thin spiral ribs and form knitted structures throughout the shell surface. The sculpture pattern is similar to G. everetti but not oblique, and the radial sculpture is more raised in G. similis. The shell shape is similar to G. corrugata and G. xesta, but these species differ entirely in their shell and protoconch sculpture (G. corrugata has irregular radial shell sculpture and straight-line protoconch sculpture; G. xesta does not have radial sculpture and the protoconch sculpture is a mix of irregular shapes).
Distribution. Georissa similis is widely distributed in the east of Sabah, from Sandakan in the north to Tawau in the south and Lahad Datu in the east. Molecular analysis. ML and Bayesian analyses of G. similis (16S: n = 5; CO1: n = 5) show that G. similis form one clade with 100% BS and 100% PP, sister to the group of G. xesta + G. nephrostoma + G. bangueyensis + G. flavescens.
Discussion. Uribe et al. (2016) have published the mitochondrial genome of G. similis (GenBank acc. no. KU342664) which was previously identified as G. bangueyensis (see phylogenetic trees, Fig. 2A and B). Phylogenetic analyses have shown that it is possible to identify the identity of a Georissa even when shell data are not available.   Diagnosis. Georissa xesta has densely arranged spiral sculpture (8-10 ribs in every 0.1 mm), unlike G. bangueyensis (4-5 ribs in every 0.1 mm), which has more space in between the spiral ribs. The shell of G. xesta looks superficially smooth under a stereomicroscope at low contrast with less than × 20 magnification. The dense spiral sculpture is similar to the spiral ribbing pattern of G. williamsi, but the shell habitus of these two species is entirely different, where G. williamsi has a broad ultimate whorl but G. xesta does not. Based on the shell shape, G. xesta is similar to G. similis and G. corrugata, but both of these species have strongly raised radial sculpture.
Discussion. The type series of G. xesta from NHMUK seems to be partially eroded. However, the densely arranged thin spiral sculpture which is the diagnostic character of the species is still visible in the type series.   Description. Protoconch. Colour: red to brown. Sculpture: rounded, ellipsoidal to irregular sculptural shape. Mesh width: 2-10 µm. Teleoconch. Colour: red to brown. First whorl: rounded to convex. Subsequent whorls: rounded to convex. Suture: clearly impressed. Shoulder: narrow. Number of whorls: 2 ½-2 ¾. SH: 0.87-1.24 mm. SW: 0.69-0.92 mm. SI: 1.26-1.43. Shell sculpture. Radial sculpture: absent, densely spaced weak to strong growth lines, no formation of true radial ribs. Spiral sculpture: present, appearing immediately after the protoconch; the ribs are low but narrow to broadly sculpted, regularly spaced, wavy, ca. 12-14 spiral ribs at the upper part of the body whorl; near the aperture, the spiral sculpture is weakened and flattened approaching the columellar region. Aperture. Shape: semi-elliptic, highly convex, and bulky parietal side, palatal side rounded, palatal edge contiguous with the parietal side, basal side convex. AH: 0.40-0.55 mm. AW: 0.43-0.60 mm. AI: 0.92-0.95.
Diagnosis. Georissa nephrostoma is characterised by the wavy formation of the spiral sculpture and the inflated parietal side of the aperture. The wavy sculpture pattern of G. nephrostoma is similar to G. flavescens but the two species differ entirely based on the shell habitus, where G. flavescens has a more compressed shell habitus. In shell shape, G. nephrostoma resembles G. similis, G. xesta and G. bangueyensis, but none of these species have the uniquely inflated parietal side of the aperture.

Georissa bangueyensis Smith, 1895
Diagnosis. Georissa flavescens is characterised by the wavy spiral sculpture, which it only shares with G. nephrostoma, but the latter species, with its narrow spire and inflated columella, is entirely distinct in shell habitus. The shell shape of G. flavescens is similar to G. gomantonensis, G. williamsi, and G. everetti, but its size is reduced compared to these three species.
Discussion. Thompson and Dance (1983) synonymised G. flavescens to G. gomantonensis, without stating any reason. We find otherwise, that G. flavescens is a valid species based on detailed conchology and molecular analysis.
Planning Unit (Prime Minister's Office, Malaysia), Sarawak Forest Department, and Sabah Biodiversity Centre for the fieldwork permits; heads of villages and local field guides for their various assistances; KNAW Ecologie Fonds and Treub Foundation for the fieldwork funds to the first and second authors. The first author would like to thank the Ministry of Education, Malaysia for the PhD scholarship award to Naturalis Biodiversity Centre and Leiden University, The Netherlands.