Sinoxychilus melanoleucus gen. nov. and sp. nov.

Protoconch with intercrossing radial wrinkles and spiral grooves. Penis sheath developed, more or less wrapping partial epiphallus. Tubercles of broken longitudinal penial pilasters bearing spinelets. Penial retractor muscle inserting on the top of penial caecum. Neither flagellum nor epiphallic papilla present. Perivaginal gland present on vagina and proximal bursa copulatrix duct.

Shell depressed; thin; opaque; of about 4.5 whorls. Umbilicus moderately wide. Protoconch with intercrossing radial wrinkles and spiral grooves. Teleoconch with spiral furrows. Aperture somewhat sinuate at peristome. Aperture toothless, unexpanded.

Sole tripartite. Caudal foss or caudal horn absent. Jaw oxygnathous, with median projection.

Penis sheath present; wrapping partial epiphallus. Penis moderately long and thick; externally simple. Sarcobelum absent. Penial caecum present, having no external demarcation between it and penis. Penial retractor muscle inserting on top of penial caecum. Flagellum absent. Epiphallus thin. Penial caecum internally with transversal ridges near epiphallic pore. Epiphallic papilla absent. Penis internally with developed pilasters. Penial pilasters broken into connected tubercles that each bearing a very short spinelet. Vagina short, internally simple, and without papilla or verge. Perivaginal gland well developed on the surface of vagina and proximal part of bursa copulatrix duct.

China (Sichuan, Hunan, Hubei).

The generic name is a compound of Greek “sino” (= China) and Oxychilus which is a genus of the family Oxychilidae.

The examined ITS2 sequences are from GenBank and this study. According to Hausdorf (2000), Gastrodontoidea is made up of six families, namely Pristilomatidae Cockerell, 1891, Chronidae Thiele, 1931, EuconulidaeH.B. Baker, 1928, Trochomorphidae Möllendorff, 1890, Gastrodontidae Tryon, 1866, and Oxychilidae. After searching for ITS2 sequences from these six families in NCBI (https://www.ncbi.nlm.nih.gov/), 21 ITS2 haplotypes of Euconulus spp. (Euconulidae), Oxychilus spp. (Oxychilidae), and one Vitrea species (Pristilomatidae) were added to our analyses (Table 1). After eliminating poorly aligned positions and divergent regions of the alignment, a dataset of 25 × 552 bp was used for the subsequent analyses. The “T92 (Tamura 3-parameter) + G” model was chosen as the best nucleotide substitution model because of the lowest AIC score (lnL = −1746.871, AICc = 3594.112). The phylograms produced by the Maximum Likelihood Inference and the Bayesian Inference are topologically identical (Fig. 8). The obtained phylogenetic inference shows Sinoxychilus gen. nov. forms a sister group with the genus Oxychilus, and both genera are well embedded in the Gastrodontoidea clade (Fig. 8).

Morphologically, this group belongs to the family Oxychilidae based on the presence of a tripartite sole, oxygnathous jaw, penis sheath, and perivaginal gland and the absence of a caudal horn and sarcobelum, by which Sinoxychilus gen. nov. can be promptly distinguished from Gastrodontidae and Pristilomatidae, the other two families of Gastrodontoidea (sensu Bouchet et al. 2017). The new genus and Oxychilus have many characteristics in common, such as a developed penial caecum, connection of some part of epiphallus + vas deferens and distal penis sheath by connective tissue, as in the European Oxychilus mortilleti (L. Pfeiffer, 1859) (Manganelli and Giusti 1998: figs 5, 10, 13, 14) and in the Asian Araboxychilus sabaeus (Martens, 1889) (Colville and Riedel 1998: fig. 7). However, Sinoxychilus gen. nov. differs from Oxychilus in having an opaque shell with a delicately sculptured protoconch, and in bearing short spinelets on the penial pilasters. The new genus also shows an unusual shell shape, which differs from shells of Ariophanta Desmoulins, 1829 and some other oxychiline genera (Fig. 7).

Zonites scrobiculatus scrobiculatus Gredler, 1885 and Z. scrobiculatus hupeina Gredler, 1887 are included in the new genus although they are only known conchologically (see Taxonomic remarks below).

Riedeliconcha Schileyko, 2003 and Vitrinoxychilus Riedel, 1963 are two oxychilid genera which also have spines on the penis inner wall. The new genus differs from them in possessing well-developed penial caecum, penis sheath, and epiphallus, a long bursa copulatrix, and conchologically, an opaque shells with a sculptured protoconch.

Holotype, 1 fma (HBUMM08236 specimen-1), Qingchengshan Mt., humid forest, in litter (Fig. 9); Sichuan Province, China; 30.919N, 103.494E; 24 March 2018; coll. Liu, Zhengping & Ma, Hongwen. Paratypes, 3 fma (HBUMM08236; specimens 2–4) and 2 fully mature empty shells (HBUMM08236; specimens 6, 7), collection data as holotype. One empty shell specimen (HBUMM08236; specimen 5) was broken accidentally after measurement and as a result is not included as a paratype. From each of type specimens with soft parts (HBUMM08236; specimens 1–4) a piece of foot (HBUMM08236a; specimens 1–4) was cut and preserved in 99.7% ethanol at −20 °C.

Shell (Figs 2, 3). Dextral; clearly depressed; very thin and fragile; opaque. Whorls convex. Suture impressed. Umbilicus moderately wide. Basal-umbilicus transition gentle. Columella arched to oblique. Columellar lip not dilated, never covering umbilicus. Protoconch with intercrossing radial wrinkles and spiral grooves (Fig. 3A). Teleoconch with regularly, densely distributed spiral furrows (Fig. 3B). Growth lines fine, distinct. Aperture large, oblique, somewhat sinuate at peristome. Body whorl straight. Adult shell neither hairy nor scaly. Body whorl of adult shell very bluntly angulate at periphery, with base convex. Aperture toothless, unexpanded. Peristome rather thin. Callus indistinct. Shell in uniformly greenish yellow, spiral band absent (Fig. 2). Measurements (n = 6): shell height = 6.7–8.1 (7.7 ± 0.55) mm, shell breadth = 12.6–13.8 (13.2 ± 0.51) mm, aperture height = 4.9–5.7 (5.3 ± 0.31) mm, aperture width = 2.2–2.6 (2.4 ± 0.16) mm, embryonic shell whorls = 1.38–1.63 (1.50 ± 0.079) mm, whorls = 4.25–4.63 (4.41 ± 0.151) mm, shell height/breadth ratio = 0.53–0.62 (0.58 ± 0.030) mm.

General anatomy. Sole tripartite. Caudal foss or caudal horn absent. Eversible head wart between ommatophore insertions absent. Tentacles and dorsum leaden-black. After preservation in 70% ethanol, black pigments on animal become faint. Lower sides and sole creamy white (Figs 4A, 10). Jaw oxygnathous, with an evidently median projection (Fig. 4B).

Genitalia (Figs 4C, 5, 6). Penis sheath about half length of penis, in holotype and two paratypes wrapping about 1/3 central epiphallus (Figs 4C, 6), but in one specimen (HBUMM08236; specimen 2) median part of epiphallus loosely joined to distal penis sheath by connective tissue (Fig. 5A). Penis more or less long, moderately thick, surface simple. Sarcobelum absent. Penial caecum present (Figs 4C, 5A, 5B, 6), having no external demarcation between it and penis (Figs 4C, 5A, 6). Penial retractor muscle inserting on top of penial caecum. Flagellum absent. Epiphallus thin, but 2–3 times thicker than vas deferens (Figs 4C, 5A, 6). Distal part of epiphallus attached at lateral side of penis by connective tissue before entering it. Penial caecum internally with three pairs of symmetrically arranged low transversal ridges near epiphallic pore which is surrounded by several very fine pilasters (Fig. 5B). Epiphallic papilla absent. Penis internally with a thickened, ‘M’-shaped median pilaster which has two arms branching into several narrow pilasters and the median pilaster running to the most proximal part of penis where it extends and forming a transversal ridge (Fig. 5B). The ‘M’-shaped median pilaster consists of connected tubercles, the apex of each bearing a very short spinelet that without exception points to atrium (Fig. 5C). Vagina short, internally simple, without papilla/verge. Perivaginal gland well developed on surface of vagina and proximal part of bursa copulatrix duct (Figs 4C, 5A). Measurements of holotype: P = 5.0 mm; Ep = 8.4 mm; VD = 6.5 mm; PR = 2.3 mm; Va = 2.3 mm; BC + BCD = 11.8 mm.

The new species is known only from its type locality.

The species is named for the clear demarcation between the leaden black ommatophores and dorsum and the remaining creamy white body, which is reminescent of the giant panda, Ailuropoda melanoleuca by having the color pattern of clear-cut patches of black and white (Fig. 10).

The new species was found living in extremely humid environment at type locality. In the laboratory, below 100% relative humidity, animals became active at the relatively lower temperature of 5 °C (Fig. 10) before they were totally inactive at room temperature (ca. 25 °C).

This new species can be distinguished from all other Chinese Hyalina species in the measurements of its shells (Table 2) and other features. This species, however, as kindly pointed out by Dr Barna Páll-Gergely, is obviously close to Zonites scrobiculatus Gredler, 1885, which was usually treated as a species in the bradybaenine genus Coccoglypta Pilsbry, 1895 (Páll-Gergely in press). The species can be promptly distinguished from Z. scrobiculatus, which has two subspecies, namely Z. scrobiculatus scrobiculatus [Zonites scrobiculatus Gredler, 1885a: 220–221, pl. 6, fig. 2; Tryon 1886: pl. 53, figs 12–14; Bachmann and Gredler 1894: 416 (radula); Retinella ? scrobiculata Kobelt 1899: 918, pl. 241, figs 10, 11; Coccoglypta scrobiculata scrobiculataYen 1939: 153, pl. 15, fig. 62; Coccoglypta scrobiculataZilch 1974: 211; Coccoglypta (Coccoglypta)scrobiculata scrobiculataZilch 1968: 180] and Z. scrobiculatus hupeina Gredler, 1887 [Zonites (Nanina?) scrobiculatus var. hupeinaGredler 1887b: 344–345; Coccoglypta scrobiculata hupeinaYen 1939: 153, pl. 15, fig. 63; Zilch 1974: 199; Coccoglypta (Coccoglypta) scrobiculata hupeinaZilch 1968: 180], by having a distinctly smaller shell, with fewer whorls, and a particular shell shape which is sharply divergent from that of Z. scrobiculatus (Fig. 7). Sinoxychilus melanoleucus is also geographically distant from the geographic range of Z. scrobiculatus (Fig. 1). Nevertheless, we are inclined to believe that based on shell morphology Z. scrobiculatus should belong to Sinoxychilus, although anatomical and molecular evidence is unavailable.

With respect to the genitalia, Sinoxychilus melanoleucus is similar to the Japanese Urazirochlamys doenitzii (Reinhardt, 1877) (Helicarionidae sensu Azuma 1995 and Schileyko 2002) in having the apical insertion of penial retractor and the absence of flagellum (Schileyko 2002: fig. 1600). Sinoxychilus and Urazirochlamys Habe, 1946 also share a characteristically spirally sculptured protoconch. However, the latter genus has a caudal horn (Azuma 1995: pl. 28, fig. 339), which suggests that Urazirochlamys does not belong to the Oxychilidae.

With the exception of two genera distributed in the southwestern part of the Arabian Peninsula, oxychilid snails are only known from the Western Palearctic (Neubert 1998; Schileyko 2003). The new species described herein, and its congeners, are undoubtedly the easternmost representatives of Oxychilidae, which suggests that Sinoxychilus might be an isolated group in China, remote from the main distribution area of the family.