Italy • 6 shells and 6 spirit specimens; Trento, Tropical greenhouse of the Science Museum of Trento (MUSE); 46°03'45.16"N, 11°06'50.08"E; 14 Dec. 2017; A. Benocci, G. Manganelli, D. Miserocchi leg.; GMC 47556 • 623 shells, 10 spirit specimens; same locality; 04 Jan. 2019, 10 Feb. 2019, 04 May 2019; D. Barbato, G. Bolzonella leg.; GMC 51194 • 418 shells, 2 spirit specimens; same locality; 01 Feb. 2022; D. Barbato, A. Benocci leg.; GMC 51184 • 133 shells; same locality; 02 Feb. 2023; F. Rossi leg.; GMC 57343 • 155 shells; same locality; 9 Feb. 2023; D. Barbato, A. Benocci leg.; GMC 57350 • 145 shells and 1 spirit specimen; Padua, Biodiversity Garden (Botanical Garden of Padua); 45°23'52.59"N, 11°52'50.37"E; 06 Mar. 2019; D. Barbato leg.; GMC 57373.

Shell (Figs 1–6). Dextral, small, minutely perforate to imperforate, elongate, slender, conical, rather robust, pearly off-white, glossy and sub-transparent when fresh, with 5–7 slightly convex whorls, separated by moderately deep sutures. Apex obtuse, rounded, and smooth. Last whorl ~ 1/2 of shell height. Aperture small, ~ 1/3 of shell height, obliquely pyriform, slightly prosocline. Peristome interrupted, not thickened, only slightly reflected on columella, sometimes with slightly evident callous rim on parietum; columella straight; outer margin sinuous in lateral view (approximately inverse S-shaped). Protoconch smooth; teleoconch with thin and irregular collabral growth lines. Shell dimensions: SH 5.5–6.8 mm; SD 2.2–2.6 mm; AH 1.9–2.3 mm; AW 1.2–1.4 mm.

Female distal genitalia (Figs 9–12). Free oviduct long and wide. Bursa copulatrix sac-like, oval with long slender duct (slightly longer than bursa copulatrix), sometimes initially flared. Vagina short and wide (approximately as long as free oviduct) with small lateral bulge.

Male distal genitalia (Figs 9–13, 15, 16). Vas deferens almost uniform in diameter (very thin to thin along its entire length), entering penial complex at its proximal end. Penial complex consisting of epiphallus, penial caecum and penis. Epiphallus very short. Penial caecum very short (as long as epiphallus). Penis short to rather long, almost uniformly cylindrical, undivided, with penial sheath enveloping its distal tract. Penial retractor muscle bifid, one branch inserted on proximal end of epiphallus, one branch on tip of penial caecum.

Genital atrium (Figs 9–12). Rather long.

Although Allopeas clavulinum is a well-known greenhouse snail (Kerney and Cameron 1979), its taxonomic and systematic placement is still not definitive, nor is its native range clear. The species was first described from Réunion Island (as Île Bourbon), Mascarene archipelago, in the western Indian Ocean (Potiez and Michaud 1838), but Griffiths and Florens (2006) thought it an East African species introduced into the Mascarene islands. The hypothesis that the species is native to Africa or East Africa was also maintained by Kerney and Cameron (1979), Robinson (1999), Probst (2001), Shea (2007), Cowie et al. (2008), Stanisic et al. (2010), and Foon et al. (2017). However Rowson et al. (2010) observed that Verdcourt, familiar with A. clavulinum in botanic gardens of the UK, never reported it from East Africa (e.g., Verdcourt 1983, 2000, 2006). Alternatively an Asian / south-east Asian origin was proposed by Brook et al. (2010) and Rumi et al. (2010). Support for a south-east Asian origin could come from the putative finding of a shell in the Holocene of Thailand (Robba et al. 2007) and its membership of a molecularly based monophyletic group, including other species from Sri Lanka (Fontanilla et al. 2017).

Today, Allopeas clavulinum occurs in humid tropical and subtropical lowlands across the world. It is reported from the West Indies, South America, West Indian Ocean islands, South and South-east Asia, New Guinea, Australia, and Pacific islands. Outside the tropics and subtropics it only occurs in heated greenhouses or very disturbed habitats, mainly in the northern hemisphere. Indeed it has been reported from North America, Europe, the Middle East, and New Zealand (see Table 1 for details and references). An alleged distinct subspecies Allopeas clavulinum kyotoense (Pilsbry & Hirase, 1904) is reported from Korea and Japan (Minato 1988; Noseworthy et al. 2007).

The genital anatomy of Allopeas species is poorly understood. Earlier studies have been performed on specimens of Allopeas gracile (Hutton, 1834) from Puerto Rico (Baker 1945; Baker in Pilsbry 1946) and Suriname (Gittenberger and van Bruggen 2013), Allopeas clavulinum from Scotland (Baker 1945; Baker in Pilsbry 1946) and Brazil (Marcus and Marcus 1968), Allopeas mauritianum (Pfeiffer, 1853) from Mauritius (Baker 1945; Baker in Pilsbry 1946) and Allopeas “spec. 2 and Allopeas spec. 3” from Pemba (Gittenberger and van Bruggen 2013). The overall distal genitalia organisation of our specimens (Figs 9–12) is consistent with that described in Allopeas gracile (Baker 1945: 88–89; Baker in Pilsbry 1946: 178, fig. 84.9; Gittenberger and van Bruggen 2013: 255, fig. 10), Allopeas clavulinum (Baker in Pilsbry 1946: fig. 84.6; Marcus and Marcus 1968: fig. 9), Allopeas spec. 2 (Gittenberger and van Bruggen 2013: 254–255, fig. 11) and Allopeas spec. 3 (Gittenberger and van Bruggen 2013: 255, fig. 12). However the relationships between the penial sheath and the vas deferens seem different from what was illustrated by Marcus and Marcus (1968), the only authors to describe them: according to Marcus and Marcus (1968: fig. 9) the vas deferens runs externally to the penial sheath whereas we found that it runs internally. The proximal penial complex consists of a short bulbous epiphallus and thin walled penial caecum with a branch of the penial retractor on the proximal tip of each (Figs 13, 15, 16). The epiphallus and the penial caecum are so closely juxtaposed as to resemble a usual proximal penis tip with an undivided penial retractor joined to it (Figs 9–12). This arrangement matches that already described in Allopeas gracile (Baker 1945: 88; Baker in Pilsbry 1946: 178, fig. 84.10), Allopeas clavulinum (Baker 1945: 90; Baker in Pilsbry 1946: 180, fig. 84.6; Marcus and Marcus 1968: fig. 9), Allopeas mauritianum (Baker 1945: 90; Baker in Pilsbry 1946: 180) and Allopeas spec. 2 (Gittenberger and van Bruggen 2013: 255, fig. 11b). However there seems to be some variation, especially in the structure of the penial caecum between these species: Allopeas gracile and Allopeas mauritianum have a robust penial caecum, large at the base and progressively tapering towards the tip (for Allopeas gracile, see Baker in Pilsbry 1946: fig. 84.10; Gittenberger and van Bruggen 2013; for Allopeas mauritianum, see Baker in Pilsbry 1946: fig. 84.3), whereas Allopeas clavulinum has a long slender penial caecum almost uniform in diameter, thin along its entire length (Baker in Pilsbry 1946: fig. 84.4; Marcus and Marcus 1968: fig. 9). Based on penial caecum structure, our specimens apparently do not match those assigned to Allopeas clavulinum but are more similar to those assigned to Allopeas mauritianum. As already rightly observed by Gittenberger and van Bruggen (2013), it is not clear whether these differences are due to individual variation or to phylogenetic divergence. The relationships between these taxa are also uncertain, as is whether these names have been used consistently in the literature. Although they are currently regarded as synonyms (MolluscaBase eds 2024b), their status and relationships are still not clear and may only be defined after the designation of neotypes and study of an appropriate number of populations using an integrative approach with conchological, anatomical and molecular characters. For now we consider this species according to its current concept (e.g., Horsák et al. 2020).

Allopeas clavulinum has been found in the tropical greenhouse of MUSE, where it is the commonest and most abundant achatinid species, and in the Biodiversity Garden (Botanical Garden of Padua), where it is rather uncommon. This is the first report from Italy.

Italy • 47 shells and 1 spirit specimen; Trento, Tropical greenhouse of the Science Museum of Trento (MUSE); 46°03'45.16"N, 11°06'50.08"E; 4 Jan. 2019, 10 Feb. 2019, 04 May 2019; D. Barbato, G. Bolzonella leg.; GMC 51196 • 17 shells; same locality; 01 Feb. 2022; D. Barbato, A. Benocci leg.; GMC 51187 • 1 shell; same locality; 02 Feb. 2023; F. Rossi leg.; GMC 57345 • 6 shells; same locality; 09 Feb. 2023; D. Barbato, A. Benocci leg.; GMC 57352.

Shell (Figs 17–20). Dextral, very small in size, very minutely perforate, elongate, very slender, conical, rather robust, pearly off-white, glossy or waxy and sub-transparent when fresh, with 5–6 slightly convex whorls separated by moderately deep sutures. Apex obtuse, rounded, and smooth. Last whorl ~ ½ of shell height and less convex than preceding ones. Aperture small, ~ 1/3 of shell height, obliquely pyriform, slightly prosocline. Peristome interrupted, not thickened, only slightly reflected on columella, sometimes with subtle callous rim on parietum; columella straight; outer margin sinuous in lateral view (approximately inverse S-shaped). Protoconch smooth; teleoconch with weak irregular collabral growth lines. Shell dimensions: SH 4.0–4.8 mm; SD 1.6–1.8 mm; AH 1.6–1.7 mm; AW 0.9–1.0 mm.

Female distal genitalia (Fig. 14). Free oviduct very short and wide. Bursa copulatrix sac-like, oval with short slender duct (as long as bursa copulatrix), initially not flared. Vagina very short and wide (as long as free oviduct).

Male distal genitalia (Fig. 14). Vas deferens almost uniform in diameter (very thin to thin along its entire length), entering penial complex at its proximal end. Penial complex apparently consisting only of penis. Penis short, almost uniformly cylindrical with thin short penial sheath enveloping its distal tract. Penial retractor muscle inserted on proximal end of penis.

Genital atrium (Fig. 14). Rather long.

The species was first named Helix clavulus by Férussac (1821: 52) based on specimens from Guadeloupe and then Helix goodalli by Miller (1822: 381) on specimens from near Bristol, England. Unfortunately Férussac’s name was not accompanied by a description, a definition, or an indication and so it is not available, whereas Miller’s name, extensively used until the early 20^th century (cf. Pilsbry 1906b), turned out to be a junior homonym and was replaced by Bulimus pumilus established by Pfeiffer (1840) on specimens from Cuba (cf. Pilsbry 1910). Pilsbry (1906a: 141–142) also discussed the hypothesis of Wollaston (1878: 510) that Helix goodalli was a junior synonym of Helix hannense established by Rang (1831) on specimens from the Cape Verde Peninsula, Senegal, observing: “whether this course was well-founded is a question which must remain unsettled until specimens from Rang’s original locality can be compared.” Consequently he never adopted Rang’s name for this species (cf. Pilsbry 1946: 181–182). The synonymy of the two species was reproposed by Groh (1983) based on study of the original descriptions and the literature, and has subsequently been adopted by most recent authors (e.g., von Proschwitz 1994; Cowie 1997, 2000; Chase and Robinson 2001; Bank and Menkhorst 2008; Gerber and Clark 2015; Horsák et al. 2020). Perplexity persists about the real identity of the species described by Rang. Only the dimensions, which are consistent with those of an Opeas species, support Groh’s interpretation. Otherwise the situation remains as described by Pilsbry more than a hundred years ago: type material of Rang’s species is unknown; no one has reported or studied material from the type locality, which when Rang visited it, was a small village, today englobed in the city of Dakar (where a green area, the Parc forestier et zoologique de Hann, still survives in Hann); finally Rang’s description and illustration are completely inadequate to establish the identity of the species he treated; his figure depicts a snail with shell having all the whorls quite round, whereas this species has the last whorl almost flat (incidentally Robinson (1999: table 1) considered Opeas hannense to be absent from Africa).

Opeas hannense is regarded as native to tropical America (Pilsbry 1946; Kerney and Cameron 1979; Deisler and Abbott 1984; Cowie 1997; Cowie et al. 2008; Brook et al. 2010; Miquel and Herrera 2014) where it is widespread in Central America and the West Indies. On the contrary Robinson (1999: Table 1) regarded it as native to East Asia. It has been introduced into South America, Atlantic islands, East Africa, West Indian Ocean islands, South-east Asia, and Pacific islands. It has also been reported from the mid temperate latitudes of the northern hemisphere where it only occurs in greenhouses and hothouses (see Table 2 for details and references). Since these reports are only based on shell identifications, it is not possible to exclude that some are misidentifications (e.g., Muratov 2010: fig. 28).

The genital anatomy of Opeas species was investigated by Baker (1945), Baker in Pilsbry (1946), and Gittenberger and van Bruggen (2013). Only three Opeas species have been studied: Opeas hannense (see Baker 1945: 86, as Opeas pumilum), Opeas pyrgula (Schmacker & Boettger, 1891) (see Baker 1945: 87; Baker in Pilsbry 1946: 183–184, figs 88 2, 3) and an unidentified species from Misali islet, Zanzibar (see Gittenberger and van Bruggen 2013: 251, fig. 9), but only the anatomy of Opeas pyrgula is adequately described. The distal genitalia of the only adult specimen that we have been able to study (Fig. 14) agrees with the general scheme described for these species and in particular with the features reported by Baker (1945) and Baker in Pilsbry (1946) for Opeas pumilum and Opeas pyrgula and with the description of Opeas pumilum given by Baker (1945). The major difference between the two species consists in the swelling between the base of the duct of the bursa copulatrix and the proximal vagina: well developed in Opeas pyrgula and much less enlarged in Opeas pumilum.

Thus, little continues to be known about the genital anatomy of this genus. We need to ascertain whether the proximal complex of the penis is really undivided, to understand the relationships between the vas deferens and the penial sheath and whether the different structure of the female distal genitalia is due to individual variation or to phylogenetic divergence.

Opeas hannense has only been found in the tropical greenhouse of MUSE, where it is uncommon. This is the first report from Italy.

Italy • 173 shells and 21 spirit specimens; Padua, Biodiversity Garden (Botanical Garden of Padua); 45°23'52.59"N, 11°52'50.37"E; 06 Mar. 2019; D. Barbato leg.; GMC 57373.

Shell (Figs 21–24). Dextral, medium in size, minutely perforate to imperforate, elongate, slender, conical, rather robust, pearly off-white, opaque, with 7–9 slightly convex whorls, separated by rather deep and in places irregularly crenulate sutures. Apex obtuse, rounded, and smooth. Last whorl ~ 1/3 of shell height. Aperture small, ~ 1/4 of shell height, obliquely pyriform, slightly prosocline. Peristome interrupted, slightly thickened along outer margin, slightly reflected on columella, with callous rim on parietum and columella; columella straight; outer margin sinuous in lateral view (approximately inverse S-shaped). Protoconch smooth; teleoconch with evident irregular collabral striae. Shell dimensions: SH 9.4–13.1 mm; SD 2.9–3.8 mm; AH 3.0–3.5 mm; AW 1.8–2.2 mm.

Female distal genitalia (Figs 25, 27). Free oviduct long and wide. Bursa copulatrix sac-like, oval with long slender duct (slightly longer than bursa copulatrix), initially slightly flared, and medially convoluted around free oviduct. Vagina long and wide (longer than free oviduct) with huge proximal lateral bulge containing large ligula.

Male distal genitalia (Figs 25–27). Vas deferens of variable diameter (proximal tract narrow, medial tract slightly wider and final tract very narrow), entering penial complex at its proximal end. Penial complex consisting of epiphallus, penial caecum and penis. Epiphallus very short. Penial caecum very short (as long as epiphallus). Penis very long, distinctly divided into proximal and distal parts by difference in calibre; proximal part longer (almost twice distal penis), slender and thin walled; distal penis shorter (half proximal penis), thick, muscular walled and enveloped by penial sheath. Penial retractor muscle bifid, one branch inserted on proximal end of penis, the other branch on tip of penial caecum.

Genital atrium (Figs 25, 27). Short.

Paropeas achatinaceum is one of the best known subulinids thanks to the excellent anatomical study and the careful taxonomic revision by Naggs (1994). Our anatomical study (Figs 25–27) fully agrees with that of Naggs (1994).

This species is regarded as native to tropical Asia, where it occurs from Nepal and Sri Lanka to South East Asia. Outside this area it is found in Australia and Pacific islands, West Indian Ocean islands, Europe, and the West Asia (see Table 3 for details and references).

The species was found in the Biodiversity Garden (Botanical Garden of Padua), where it forms a well-established population, as in Vienna Zoo, Austria (Horsák et al. 2020). In contrast, the report from Malta is only based on a specimen of unknown origin recovered from sandy detritus collected on a beach (Cilia et al. 2022). This is the first report from Italy and the third from Europe.

Italy • 112 shells and 2 spirit specimens; Trento, Tropical greenhouse of the Science Museum of Trento (MUSE); 46°03'45.16"N, 11°06'50.08"E; 04 Jan. 2019; 10 Feb. 2019; 04 May 2019; D. Barbato, G. Bolzonella leg.; GMC 51197 • 29 shells; same locality; 01 Feb. 2022; D. Barbato, A. Benocci leg.; GMC 51188 • 1 shell; same locality; 09 Feb. 2023; D. Barbato, A. Benocci leg.; GMC 57353 • 1 shell; Padua, Biodiversity Garden (Botanical Garden of Padua); 45°23'52.59"N, 11°52'50.37"E; 06 Mar. 2019; D. Barbato leg.; GMC 57374.

Shell (Figs 7, 8, 28–30). Shell dextral, medium in size, imperforate, elongate, slender, conical, rather robust, pearly off-white, glossy or waxy and sub-transparent when fresh, with 7–9 slightly convex whorls, separated by rather deep and in places irregularly crenulate sutures. Apex obtuse, rounded, and smooth; last whorl ~ 1/3 of shell height. Aperture small, ~ 1/5 of shell height, ovate, slightly prosocline. Peristome interrupted, not thickened or reflected, with callous rim on parietum and columella; columella concave and obliquely truncate at base; outer margin straight in lateral view. Protoconch smooth; teleoconch with fine, wrinkled, irregular collabral growth lines. Shell dimensions: SH 12.1–16.0 mm; SD 3.3–4.1 mm; AH 2.5–3.2 mm; AW 1.9–2.4 mm.

Female distal genitalia (Fig. 31). Free oviduct very short and wide. Bursa copulatrix sac-like, oval with short slender duct (as long as bursa copulatrix), initially not flared. Vagina very long and slender.

Male distal genitalia (Figs 31, 32). Vas deferens of varying diameter (proximal tract narrow, medial tract slightly wider and final tract very narrow), entering penial complex near proximal end. Penial complex consisting of epiphallus, penial caecum and penis. Epiphallus very short. Penial caecum very short (as long as epiphallus). Penis very long, divided distinctly into proximal and distal parts by difference in calibre, without penial sheath; proximal part longer (twice distal penis), slender and thin walled; distal penis shorter (half proximal penis) and thick, muscular walled. Penial retractor muscle bifid, one branch inserted on tip of penial flagellum, the other branch on tip of penial caecum.

Genital atrium (Fig. 31). Very short.

Subulina octona is a well-known travelling snail that occurs worldwide (Robinson 1999) but its native range is uncertain. It was described from Guadeloupe and Hispaniola and has been reported as native to tropical America (Pilsbry 1946), the Caribbean (Deisler and Abbott 1984) and Latin America (Robinson 1999) but this was disputed on the grounds that the other species of the genus occur in Africa (Bieler and Slapcinsky 2000). Gerlach (2006) regarded it as native to the Seychelles, based on a subfossil record from Aldabra reported by Gerlach and van Bruggen (1999). Others suggested that it was originally from tropical America (Griffiths and Florens 2006), south-east Asia (Rumi et al. 2010) or Africa (Breure et al. 2016; Hovestadt and van Leeuwen 2017). Multi-gene phylogenetic analysis of the achatinoid snails by Fontanilla et al. (2017) found that Subulina octona and Subulina striatella (Rang, 1831) formed a monophyletic group belonging to an unsupported clade including only Old World species, but unfortunately the study included a small selection of subulinine genera, only one of which was from neotropical America.

Subulina octona is now distributed widely in humid tropical and subtropical lowlands across the world. It occurs in Central America, the West Indies, South America, sub-Saharan Africa, West Indian Ocean islands, South, South-East, and East Asia, New Guinea, Australia, and Pacific islands. In the mid temperate latitudes of the northern hemisphere, it only occurs in greenhouses and hothouses (see Table 4 for details and references).

The distal genitalia of MUSE specimens show a general scheme identical to that described by Baker (1927: 3–4, pl. 20, fig. 99), Marcus and Marcus (1968: 189–190) and Araújo and Bessa (1993: 493–495, figs 7–11), but differing in the size of the penial complex from that described by Wiegmann (1894: 214–216, pl. 16, fig. 3; reproduced by Pilsbry 1946: 173, fig. 83h), Schileyko (1999: fig. 662 BC) and Gittenberger and van Bruggen (2013: 250). Instead of a very short penial complex, in agreement with the first description by Baker (1927), MUSE specimens have a long slender penial complex consisting of a short proximal portion (the epiphallus and the penial caecum closely juxtaposed to each other), a long slender medial portion (the proximal penis) and a short swollen distal portion (the distal penis). In particular, based on the male genitalia of a specimen from Dunoon (Guyana) mounted in glycerine jelly and viewed by transmitted light, Baker (1927) described the proximal portion as consisting of a flagellar appendix [the penial caecum] and a very short, thick walled tract [the epiphallus]; the vas deferens entering the penial complex at the proximal end of the epiphallus; the penial retractor joining at the tip of penial caecum; a rather elongate papilla present at the internal opening of the epiphallus into the proximal penis. Baker also described a penial sheath enveloping the distal penis joined by a muscular branch originating from the right lower tentacle retractor; finally he interpreted the structure that Wiegmann (1894) described and figured as the penis as being only the distal penis surrounded by a heavy muscular sheath.

The small size and the very fine structure of the sections of the proximal portion of the penial complex make dissection difficult and differentiation of its components elusive. Our results substantially agree with Baker’s description. The differences are: the vas deferens enters the penial complex near the base and not at the tip of the epiphallus; the penial retractor consists of two branches, one joined to the epiphallus, the other to the penial caecum; the small size of the proximal penis makes the penial papilla impossible to detect by stereomicroscope; a classical penial sheath (such as that of Allopeas and Paropeas) is absent, although a muscular branch from the right lower tentacle retractor joins the distal penis directly on the penial wall. Unfortunately, the scarcity of material prevented a more careful anatomical examination. Some uncertainties about its real organisation and the meaning of such differences among the various anatomical reports therefore remain.

Multi-gene phylogenetic analysis of the achatinoid snails by Fontanilla et al. (2017) found that the genus, as currently conceived, is polyphyletic because the three Subulina species examined did not cluster together. The meaning of variation in penis size in Subulina octona is also uncertain. Wiegmann (1894) ventured that the extreme reduction of the male distal genitalia was related to “unisexuality” or sequential hermaphroditism (... dass die betreffenden Thiere durch Verkümmerung des männlichen Theils der Genitalien eingeschlechtig geworden sind, oder aber dass die weibliche Geschlechtsreife der männlichen vorausgeht). Others, such as Baker (1927), supposed that the species was a sequential hermaphrodite, but whereas Wiegmann saw it as protogynous, Baker viewed it as protandrous. Subulina octona is actually a facultatively self-fertilising egg-retaining species showing no evidence of sequential hermaphroditism (Bessa and Araújo 1995; D’Ávila et al. 2018). Thus the reduced penis may be related to loss of biparental reproduction as already supposed by Wiegmann. However this may be because Subulina octona is a complex of species. More research is needed to address these questions.

Subulina octona is common in the tropical greenhouse of MUSE, and has been found in the Biodiversity Garden (Botanical Garden of Padua), where only one specimen was collected. This is the first report from Italy.

Italy • 2 shells; Trento, Tropical greenhouse of the Science Museum of Trento (MUSE); 46°03'45.16"N, 11°06'50.08"E; 01 Feb. 2022; D. Barbato, A. Benocci leg.; GMC 51189.

Shell (Figs 33, 34). Dextral, very small in size, imperforate, elongate, very slender, cylindro-conical, thin and fragile, pearly off-white, colourless, glossy and transparent when fresh, ~ 4 slightly convex whorls separated by rather deep sutures. Apex obtuse, rounded, and smooth. Last whorl ~ 2/3 of shell height. Aperture small, ~ 1/3 of shell height, ovate to pyriform, basally flared, slightly prosocline. Peristome interrupted, not thickened or reflected, with callous rim on parietum and columella; columella straight or slightly concave; outer margin slightly arched forward in the middle in lateral view. Protoconch smooth; teleoconch with very fine collabral lines and very fine spiral grooves particularly evident on last whorl. Shell dimensions: SH 2.8 mm; SD 1.0 mm; AH 1.1 mm; AW 0.6 mm.

Body and anatomy. Unknown.

The early taxonomy of this land snail revolves around three named species: Macrospira aperta Swainson, 1840, Achatina gundlachi Pfeiffer, 1850 and Geostilbia caledonica Crosse, 1867.

Commenting on the land mollusc species introduced to Saint Helena, Edgar Smith maintained that Achatina gundlachi was a junior synonym of Macrospira aperta based on examination of Guilding’s material from St. Vincent, West Indies, deposited in the British Museum (Smith 1892) and that Geostilbia caledonica was also a junior synonym of Megaspira [sic] aperta (Smith, 1895).

Pilsbry (1908), in his exhaustive revision of orthurethrous snails included in the second edition of the Manual of Conchology, partly rejected Smith’s conclusions, regarding Macrospira aperta as a species inadequately described and Geostilbia as a section of Cecilioides. He used Cecilioides gundlachi as the valid name for the species. However many years later, dealing with this group again, he adopted Cecilioides aperta as the valid name (Pilsbry 1946).

No subsequent authors made any significant contribution for a better taxonomic framework of the species. They repeated what Pilsbry (1946) proposed, believing that a species of Geostilbia, sometimes considered a subgenus or a synonym of Cecilioides, could be found almost everywhere in the world, having spread from the Mesoamerican area (e.g., Robinson 1999). Indeed many consider it native to the West Indies, Neotropics or Caribbean basin (Dundee 1974; Cowie 1997; Robinson 1999; Chase and Robinson 2001; Rosenberg and Muratov 2006; Thompson 2011; Miquel and Herrera 2014; Nurinsiyaha et al. 2016; Nurinsiyaha and Hausdorf 2019). Others report it to be native to southern Europe (Cotton 1954) or North America north of Mexico (Nekola 2014).

A species, sometimes named Geostilbia aperta (or Cecilioides aperta), Geostilbia caledonica (or Cecilioides caledonica) or Geostilbia gundlachi (or Cecilioides gundlachi), is currently reported from North, Central and South America, the Caribbean and the Indo-Pacific Region from South-East Asia to Hawaii and Cook Islands (see Table 5 for details and references). It has also been reported from Saint Helena (Smith 1892) based on a misidentification of Cecilioides acicula (Crowley and Pain 1977), and from Barbados, Curaçao and Galapagos, where it was no longer found in recent field surveys (Chase and Robinson 2001; Miquel and Herrera 2014; Hovestadt and van Leeuwen 2017).

It is difficult to say anything new about this group of species, since there is no anatomical data. We can rely on shell characters, the most interesting of which are the spiral sculpture particularly evident in the last whorl, the non-truncated or slightly truncated columella and the basally flared aperture. Based on a close resemblance to Cecilioides, the species of Geostilbia have been placed in the family Ferussaciidae but this similarity could also be due to convergence: true Cecilioides have no microsculpture on the teleoconch, have a truncated columella and have no basally flared aperture. In their phylogenetic analysis of the achatinoideans Fontanilla et al. (2017) examined a Cecilioides species – Cecilioides gokweana (Boettger, 1870) – which may actually be Geostilbia, if the material investigated, collected by DG Herbert, matches the description of this species given by him (cf. Herbert 2010: 127 for description and figure). If this is confirmed, Geostilbia does not belong to the Ferussaciidae but to a distinct group of Achatinidae. This of course does not resolve the relationships between Cecilioides and Geostilbia: Cecilioides may really belong to the Ferussaciidae or to the same or a different group of Achatinidae, which may include Geostilbia.

MolluscaBase lists eight Geostilbia species (MolluscaBase eds 2024c), but except for the widespread Geostilbia aperta and the south American Geostilbia blandiana Crosse, 1880, all the others occur from Madagascar to Southeast Asia. There is great uncertainty about species-level taxonomy of Geostilbia. Some of the species listed by MolluscaBase may prove to be true Cecilioides based on the apparent absence of spiral microsculpture, truncated columella and not basally flared aperture: this could be true of Geostilbia philippinica von Möllendorff, 1890 and Geostilbia sheilae Groh, 2015 (see Groh 2015). On the other hand, some species reported as Cecilioides by MolluscaBase, such as the East African Cecilioides callipeplum (Connolly, 1923), for which clear spiral sculpture is reported by Verdcourt (1986), van Bruggen and van Goethem (2001), and van Bruggen (2008), or the South African Cecilioides gokweana, for which clear spiral sculpture is described by Herbert (2010), may prove to be true Geostilbia.

We assign two shells found in the litter of the tropical greenhouse of MUSE to this species (unfortunately no living specimen was found during our collecting). This is the first report of the species from Europe.