Squamapex taiji Zhang and von Rintelen sp. nov.

LSID urn:lsid:zoobank.org:act: 34521C1F-6673-4370- A857-75C9C818ED51.

Holotype: NACRCCAS FG_609870, collected on 27 March 27 2022, by Le-Jia Zhang from a small rock about 0.1 m deep at the western shore of Lake Fuxian near Jianshan Village, Chengjiang City, Yunnan, China.

Paratypes: NACRCCAS FG_609871 to FG_609888, 18 individuals; ZMB/Moll: 122774, two individuals; all paratypes from the same rock as the holotype.

Description: Shell (Fig. 3) minute, width 1.56 ± 0.11 (range 1.22 to 1.76) mm, height 1.04 ± 0.07 (range 0.82 to 1.12) mm, dextral, depressed cap-shaped, thin but solid, orange to reddish brown in colour; mostly with three whorls, including one-and-a-half protoconch whorls with dense pits, one-and-a-half teleoconch whorls with fine growth lines, spiral whorls short and depressed, body whorl large and wide, shell base flat; aperture kidney-shaped; outer lip thin but solid, upper, central, and lower part extended, forming a wide strongly sinuate margin with three protuberances; inner lip extremely extended, forming a flat transparent glassy semicircular structure, covering umbilicus completely and nearly three-fourth part of shell base.

Protoconch (Fig. 4A) broad, one-and-a-half whorls, low dome-shape, first half whorl with weak sparse veins, the rest one whorl with 14 to 16 rows of regularly distributed, large, square-scale-shaped pits exposed, all square-scale-shaped pits similar in size.

Operculum (N = 4) (Fig. 4C) corneous, kidney-shaped, thin and fragile, transparent yellow in colour; subcentral paucispiral nucleus visible on the exterior surface, close to the inner margin and opercular base, on the upper and lower part each with a shallow apophysis; interior surface with a long hourglass-shaped inner opercular region, on the central of the upper and lower part of the inner opercular region each with a projection, oblong upper projection obviously longer than square lower projection, the lower end of upper projection and upper end of lower projection both pointing to the central of inner margin; enamel region flat and smooth, inner enamel region narrow and thickened; marginal region very thin and fragile, filling the concave region of central inner margin.

Radula (N = 2) (Fig. 4B) taenioglossate (2 + 1 + 1 + 1 + 2); central teeth trident-head-shaped, with two deep, lower concave edges, upper edge with three sharp triangle cusps—one smaller cusp on either side of a big and long median cusp, with one long, large, basal denticle on the central part of either lower concave edge; lateral teeth with two to three cusps on the inner side and six cusps on the outer sides; inner marginal teeth with 14–19 cusps, outer marginal teeth with 10–14 cusps.

Head–foot of ethanol-preserved specimens (N = 3, all females) (Fig. 4D, E) transparent pale grey; a pair of eyebrow-shaped darker patterns on the base of each tentacle, behind each eye, another pair of eyebrow-shaped darker pattern on the base of either side of the snout, under each eye; foot small, anterior part expanded laterally and anterior edge indented.

Morphological comparison: Squamapex taiji gen. et sp. nov. has a depressed cap-shaped shell with an extremely extended inner lip that covers the base. This character is somewhat similar to the shell of Lacunopsis harmndi Poirier, 1881 from NE Thailand and Lacunopsis yunnanensis Liu et al., 1980 also from Fuxian Lake (Supporting Information, File S1- Fig. S1), both belonging to the family Pomatiopsidae Stimpson, 1865. However, both species are much larger (width of L. harmndi about 7 mm; width of L. yunnanensis about 3 mm), and have a simple outer lip, a smooth protoconch and a simple oval operculum; the radula, especially the central teeth and more whorls (4.5), of L. yunnanensis is distinct from those of Squamapex taiji gen. et sp. nov. as well.

Ecology and distribution: Densely inhabiting rocks at the strongly wave-exposed lake shore; living animals currently are only known from Fuxian Lake, Yunnan, China. The living animals were very active and moved fast in the aquarium.

Etymology: The overall shape of the inner lip and aperture is similar to the Chinese yin-yang symbol, Taiji. Formed as a noun in apposition. The recommended Chinese name is ‘kDz üūDz’.