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.