Calybium, Morlet, 1892

Hoang, Ngoc Khac, Vu, Van Lien, Tran, Thi Thanh Binh, Nguyen, Thanh Son & Do, Duc Sang, 2025, Description of the second Calybium Morlet, 1892, and the first record of the genus for Vietnam’s terrestrial gastropod fauna (Neritimorpha: Helicinoidea: Helicinidae), Ruthenica, Russian Malacological Journal (New York) 35 (1), pp. 39-45 : 41-44

Calybium plicatus sp. nov.

( Figs 1 , 3–5 )

Zoobank registration: urn:lsid:zoobank.org:act:

D7B00C79-E41F-4274-9F98-E468B1B97C6E

Type material. Holotype, VNMN-IZ 000.002.347, Vietnam, Quang Binh Province, Bo Trach District, Tan Trach Commune, Phong Nha – Ke Bang National Park , Doline 1 area of Son Doong Cave (17°27.51.1” N 106° 17.12.7”E), coll. Hoang N.K., 14 January 2023 . Paratype: ZVNU. MOL 049 (01 shell with incomplete aperture, and part of last whorl is broken), same data as holotype .

Diagnosis. Shell medium-sized, depressed conical, and sharply keeled body whorl; the calcareous layer on the dorsal side is coarse, strongly undulated and forms evenly spaced domes protruding from the shell surface; parietal wall with six lamellae, of which the sixth very long, curved, and close to the suture; palatal wall with a barrier-shaped lamella that parallel with the parietal lamellae; basal wall with a sickle-shaped lamella with its upper part long and forms ridge and lower part raised a knob-like.

Description. Shell medium-sized, thin, depressed conical, with a relatively acute spire, consists of 6¼–6½ whorls; protoconch consists of 1.5 whorls, protruding above first whorls of teleoconch, pale yel- low to chocolate brown, glossy, seemingly smooth, irregularly ribbed, the ribs are more prominent at the lower part of the whorls; teleoconch consists of 5.0 slowly increasing whorls; suture shallow and not visible. The surface of the teleoconch has a thin, coarse, and calcareous spiral layer, which also forms numerous domes that are regularly spaced and protrude from the surface shell. Furthermore, this calcareous layer extends and sharpness almost completely covers the suture. The last whorl has a very sharp peripheral keel, which is widened and wavy quite regularly like a curtain, the keel with darker color than other parts of the shell; the apical side is covered by rough, irregular wrinkles, usually stronger at the periphery of whorls, and between the main wrinkles there are fine radial lines; umbilical side usually covered by regularly arranged riblets of ribs, weaker at the around the callus. Aperture parallelogram, peristome discontinuous, thickened, expanded, and reflected; parietal callus thin and opal- escent. Apertural denticles are always present, very large, and free from the callus. There are 6 lamellae on the parietal wall, with the most prominent part being the sixth one, which is very long, curved, and close to the suture; the first lamella is the smallest, inverted C-shaped, and located close to the columel- lar side; 3 middle ones are relatively similar in shape and size, and all smaller than the second one; the fifth one is strong and hook-like shape. A very large and barrier-shaped lamella on the palatal wall, which extends and runs parallel with the parietal lamellae. In the basal wall, there is a sickle-shaped lamella with its lower part long and forms a ridge, the upper raised a knob-like. Umbilicus is always completely closed.

Measurements of holotype (in mm). Shell height 13.1, shell width 7.0, aperture height 2.9, aperture width 6.9.

Distribution. This species is only known from its type locality. However, we assume this species may occur locally in the limestone mountain range on the Vietnam-Laos border, especially in limestone caves.

Etymology. The species’ name is derived from its characteristic apertural dentition, from the Latin word ‘plicatus’, meaning ‘lamellae’ that are present at the parietal wall.

Ecology. This species was found on the surface of a large limestone rock in the cave, which had moss, fern, aerophytic algae, and cyanobacteria. In addition, the Son Doong Cave has environmental factors such as light intensity, temperature, and humidity that are often different from those outside (see Fig 5).

Remarks. The characters distinguishing Calybium plicatus sp. nov. from C. massiei are the smaller shell with a more elevated spire, the parietal wall with six distinguished and evenly spaced lamellae, of which the sixth one is very long with the tip ap- proaching the callus, the fifth one is strong but does not extend to the callus, a large palatal lamella as the barrier, and a very long basal lamella, which is sickle-shaped with upper part long and lower part raised a knob-like.

The finding of a second species in this study suggests that Calybium species can be found in the border area between central Vietnam and central Laos, especially limestone areas such as forests over limestone, caves, and isolated karst. In addition, both C. massiei and Calybium plicatus sp. nov. were discovered in stable environmental conditions of caves, which may suggest that Calybium species are relict species of extinct lineages.

Figures

FIG. 1. Distribution of Calybium Morlet, 1892: blue circle – Calybium massiei Morlet, 1892; red triangle – Calybium plicatus sp. nov.

FIG.3. Calybium plicatus sp. nov. A–F. Holotype VNMN-IZ 000.002.347.A–D. Shell morphology (apertural view, dorsal view, ventral view, and side view). E–F. Details of the apertural dentition. Photos: Nguyen T.S. and Do D.S. РИС. 3. Calybium plicatus sp. nov. A–F. Голотип VNMN-IZ 000.002.347. A–D. МорфологиЯ раковины (вид спереди, вид сверху, вид сниЗу и вид сбоку). E–F. Детали устЬевой арматуры. Фотографии: Нгуен Т.С. и До Д.С.

FIG. 4. Morphology of apertural dentition of Calybium plicatus sp. nov. A. Parietal lamellae. B. Palatal lamella. C. Basal lamella. 1–6: Determining the order of lamellae on the parietal wall.