Biomphalodonta forticostata Herbert 2020, gen. et sp. nov.

Biomphalodonta forticostata gen. et sp. nov.

urn:lsid:zoobank.org:act: 83C8FFD5-F9B7-493E-A4E6-2E14B7B2AF98

Figs 8 I–O, 9, 18G–H

Diagnosis

Shell relatively large, biconcave with deep, tightly coiled whorls; spire deeply sunken; protoconch sculptured by fine axial riblets; teleoconch with relatively strong, widely spaced axial ribs, their intervals with microscopic axial and even finer spiral threads; aperture narrow, crescent-shaped, lacking parietal and columellar dentition; palatal region with 1–2 axially aligned rows of small denticles set back ¼ and ½ whorl behind outer lip, visible only by transparency; rows broadly C-shaped, each with up to 6 denticles; umbilicus wide and deep, V-shaped. Shell somewhat translucent, typically pale straw-brown, but occasional specimens milky-white; diameter up to 2.7 mm.

Etymology

From the Latin fortis: strong and costa, costata: a rib, ribbed; with reference to the coarse axial sculpture.

Material examined

Holotype

SOUTH AFRICA • KwaZulu-Natal, Ngele Forest; 30.533° S, 29.633° E; 1350 m a.s.l.; 3 Mar. 1997; D.G. Herbert leg.; mistbelt Podocarpus forest, sorted from dried leaf-litter sample; diameter 2.13 mm, height 1.20 mm; NMSA V6949/T4261.

Paratypes

SOUTH AFRICA – KwaZulu-Natal • 7 specimens; same collection data as for holotype; NMSA V4886/ T4262 • 1 specimen; Hlabeni Forest; 29.975° S, 29.742° E; 1350 m a.s.l.; 24 Aug. 1997; C. Symes leg.; mistbelt Podocarpus forest, in leaf-litter sample; NMSA V5221/T4263 • 1 specimen; Ngele Forest; 30.525° S, 29.692° E; 1300 m a.s.l.; 14 Nov. 1995; L.S. Davis and D.G. Herbert leg.; mistbelt Podocarpus forest, in leaf-litter; NMSA V2069/T4260.

Other material

SOUTH AFRICA – KwaZulu-Natal • 1 specimen; Creighton area, Riverside, Kwahoha Forest; 30.1472° S, 29.52778° E; ca 1300 m a.s.l.; 27 Jan. 2001; C. Symes leg.; mistbelt Podocarpus forest, in leaf-litter; NMSA V9173 • 1 specimen; Ngele Forest; 30.533° S, 29.633° E; 1350 m a.s.l.; 14 Nov. 1995; D.G. Herbert and L.S. Davis leg.; mistbelt Podocarpus forest, in leaf-litter; NMSA V5187.

Description

Shell relatively large, diameter up to 2.7 mm, H/D ratio ±0.56; biconcave with deep, tightly coiled whorls; spire deeply sunken; last adult whorl not descendant; suture strongly indented, and apical and basal portions of whorls strongly convex, peripheral portion less so; largest individuals with a shallow supra-peripheral indentation in second half of last adult whorl and a weakly angled periphery (Fig. 8 M–N). Protoconch comprising apical cap plus 0.75 whorl; diameter ±340 μm; sculptured by fine axial riblets, relatively widely spaced on apical cap, becoming progressively more close-set toward junction with teleoconch. Teleoconch of up to 4.0 whorls, its sculpture coarse, comprising relatively strong, widely spaced axial ribs, their intervals with close-set microscopic axial threads; even finer, close-set microscopic spiral threads also evident in fresh specimens; axial ribs for the most part regularly spaced, but becoming less regular, more close-set and noticeably sinuous in last quarter whorl of largest specimens. Umbilicus wide and deep, V-shaped, but suture indented and whorl margins strongly convex. Aperture narrow, crescent-shaped, but apical and basal limits rounded; parietal and columellar dentition lacking; palatal region with 1–2 axially aligned rows of small denticles set back ¼ and ½ whorl behind outer lip, not visible through aperture and apparent only through translucent shell; each row with up to 6 denticles; number and alignment of denticles somewhat variable and related to degree of development; those nearest periphery usually slightly more anterior and rows thus broadly C-shaped at full development. Shell somewhat translucent, typically pale straw-brown, but occasional specimens milky-white.

Distribution and conservation

A narrow-range endemic (Fig. 9), known only from south-western KwaZulu-Natal, between Kokstad and Donnybrook, at 1300–1400 m a.s.l.; living in leaf-litter of southern mistbelt forest. Although forests in this area are theoretically protected, access is completely uncontrolled and they are often in close proximity to exotic timber plantations and thus exposed to threats associated with disturbance and alien plant invasion.

Remarks

The relatively large and deeply biconcave shell of this species resembles those of Amatholedonta bimunita gen. et comb. nov. and Am. fordycei gen. et sp. nov. The coarse axial sculpture of the present species, however, renders it distinctive. Additionally, in Am. bimunita gen. et comb. nov. the palatal dentition, as seen by transparency, comprises 1–3 more or less vertical rows of five denticles (occasionally four), the lower three in each row more elongate. A minute sixth denticle is also present at the junction of the basal and columellar lips, but is easily overlooked. In Am. fordycei gen. et sp. nov. there are similarly 1–3 axially aligned rows of denticles, but each has only three denticles, the lowest of which is markedly more elongate. In neither Am. bimunita gen. et comb. nov. nor Am. fordycei gen. et sp. nov. is the protoconch axially ribbed. With a maximum diameter of 2.7 mm, B. forticostata gen. et sp. nov. is the largest species of dentate charopid snail known to date from southern Africa.