Tropidophora sericea, Griffiths & Herbert, 2013

Griffiths, O. L. & Herbert, D. G., 2013, New species of land snails (Mollusca: Gastropoda) from two isolated karst formations in central western Madagascar: Tsingy Beanka and Antsingimavo, with additional notes on other regional endemics, African Invertebrates 54 (1), pp. 1-48 : 13-14

Tropidophora sericea sp. n.

Fig. 9

Etymology: From Latin sericeus (silky); in reference to the silky texture of the shell.

Diagnosis: Spire very low, shell almost planorboid, body whorl not conspicuously tumescent, umbilicus very wide; columella lip weakly reflected, but not broadly expanded over umbilicus; sculpture finely decussate, comprising low, close-set spiral ridges crossed by similar axial pliculae; yellowish white with traces of fine orange-brown spiral lines.

Description:

Shell: Medium to large, depressed-discoidal to planorboid, spire very low with little other than embryonic whorls projecting in apertural view; final part of last adult whorl descending gently prior to aperture; whorls more or less evenly rounded, suture indented; umbilicus very wide, its margin evenly rounded; underside of embryonic whorls clearly visible. Protoconch of 1¼–1½ whorls, essentially smooth, but microscopically shagreened. Teleoconch of a further 2¾–3 whorls; first whorl initially with 6 or 7 low spiral ridges increasing to double this by end of whorl; ridges crossed by numerous, close-set axial pliculae producing a finely decussate sculpture; intervals between ridges 2–3 times wider than those between pliculae; subsequent whorls with additional spiral ridges arising by intercalation, but sculpture remaining finely decussate more or less throughout, including base; spiral ridges more close-set on later whorls and sculptural interstices less axially elongate; ridges with minute granules where crossed by axial pliculae. Aperture subcircular, strongly oblique to vertical axis of shell; peristome virtually complete, somewhat angled in parietal region; aperture expanding slightly prior to lip and lip also weakly reflected.

Ground colour dirty white to yellowish buff; the freshest specimens retaining evidence of a pattern of fine orange-brown spiral lines above the periphery, that at periphery strongest; similar lines just below periphery but umbilicus and most of base lacking colour pattern.

Dimensions: Holotype, max. diameter 37.3 mm, height 18.9 mm; largest specimen, max. diameter 41.4 mm.

Holotype: MADAGASCAR: Central W Madagascar, ca 60 km E of Maintirano, NE of Belitsaka , central part of Tsingy Beanka , in slots and small caves above Bokarano R. gorge, next to low dry deciduous forest, 17.90517°S 44.48240°E, ca 215 m, 22.ix.2010, O. Griffiths & R. Randalana, st’n 09/10 ( AMS C.474166) . Paratypes: Same data as holotype ( NMSA L8524 /T2979, 3 specimens); st’n 06/09 ( AMS C.469585, 3 adult specimens); st’n 07/09 ( TMAM T161 , 1 adult specimen); st’n 08/09 ( AMS C.469586, 9 adult specimens); st’n 08/10 ( MHNP IM-2010-20083, 4 adult specimens) .

Additional locality data: Tsingy Beanka : st’ns 06/10, 07/10, 09/12.

Distribution: A narrow-range endemic; currently known only from the central region of the Tsingy Beanka .

Habitat: Found only in subfossil form in deep slots and crevices within the tsingy, in dry deciduous forest.

Remarks: Like T. humbug , this species is referable to the ‘groupe d’espèces du T. deshayesiana (Petit de la Saussaye, 1844) ’ of Fischer-Piette et al. (1993). Within this group, it is closest to T. chavani Fischer-Piette, 1949 (see above) in having a very low spire, relatively fine spiral sculpture and a colour pattern of narrow, brownish, spiral bands. It is, however, considerably larger than T. chavani , attaining as much as 41.4mm in max. diameter (compared to 32 mm for T. chavani ). Furthermore, its sculpture is considerably finer than that of T. chavani , with subequal spiral and axial elements. In T. chavani the axial pliculae are crisp, and much finer and more close-set than the spiral cords (compare Figs 9D and 9E ).At some localities, T. chavani occurs in subfossil form together with T. sericea , but the two remain clearly distinct.

T. moulinsii (Grateloup, 1840) ( Fig. 26A–C , holotype, NHMUK) and T. thesauri Fischer-Piette, 1949 ( Fig. 26D–F ), both from the north-eastern tip of Madagascar, are of a more similar size (attaining 38 mm or more in max. diameter), but T. thesauri has much deeper whorls, a higher spire, narrower umbilicus, irregular scale-like subsutural pliculae and lacks the finely decussate microsculpture of T. sericea . T. moulinsii has stronger, cord-like, spiral sculpture, more tumescent whorls and a higher spire.

Figures

Fig.9. Tropidophora sericea sp.n.and T. chavani: (A–C) T.sericea, holotype,Tsingy Beanka,max.diameter 37.3mm, height18.9 mm,AMS C.474166;(D, E) adult teleoconch sculpture of T. sericea (holotype) and T. chavani (NMSA L8447) respectively, scale bars = 2.5 mm.

Fig.26.Speciesillustrated for comparative purposes:(A–C) Tropidophora moulinsii, holotype of Cyclostoma moulinsii Grateloup, 1840, ‘Madagascar’, max. diameter 33.1 mm, NHMUK 1907.11.22.12; (D–F) Tropidophora thesauri Fischer-Piette, 1949, Orangea, Diego Suarez [= Antsiranana] district, max. diameter 38.0 mm, NMSA L8438; (G–I) Tropidophora reticulata, one of three syntypes of Cyclostoma reticulatum (Adams & Reeve, 1850), ‘Madagascar’, max.diameter 15.1 mm, NHMUK 1874.12.11.231; (J–L) Kalidos hova, syntype of Nanina hova Odhner, 1919, Catsèpe, Madagascar, max. diameter 24.4 mm (SMNH).