Orbitestella amphaengensis, Ketwetsuriya & Dumrongrojwattana, 2021

Ketwetsuriya, Chatchalerm & Dumrongrojwattana, Pongrat, 2021, A new microgastropod species, Orbitestella amphaengensis, (Gastropoda: Heterobranchia: Orbitestellidae) from Bangkok clay of Samut Sakorn Province, Thailand, Raffles Bulletin of Zoology 69, pp. 304-308 : 305-307

Orbitestella amphaengensis , new species

( Fig. 3 )

Material examined. Holotype (H 0.88 mm, W 1.79 mm, AH 0.79 mm, AW 0.86 mm) (ESKU- 2021-I- 001), Thailand, Samut Sakorn Province , Ban Phaeo District (13°35′52.296″N, 100°11′29.8032″E), coll. C. Ketwetsuriya, 26 November 2020.

Etymology. After Amphaeng Sub-district, where the studied gastropod material was found.

Description. Shell dextral, minute, discoidal, tightly coiled with a flat spire and widely phaneromphalous. Suture well marked and deeply impressed. Protoconch consisting of ca. 1.5 whorls, entirely smooth and rounded without visible ornamentation (diameter 320 µm in Fig. 3B, D ). Boundary between protoconch and teleoconch conspicuously marked. Teleoconch of ca. 2 whorls; early teleoconch (first ¼ whorl) with the angulation placed just below adapical suture forming crest and one additional angulation situated at the abapical surface forming periphery; whorls sculptured by numerous strongly spaced axial ribs; spaces between axial ribs concave. Later teleoconch whorl ornamented with four prominent spiral carinas; carina 1 located close to adapical suture with row of strong nodules, continuing from crest of early teleoconch; carina 2 placed at a distance of about from the total width of the whorl, becoming from the second angulation of early teleoconch; carina 3 which is the strongest carina on mid-whorl of body whorl forming prominent peripheral keel; carina 4 angulated at transition to base. Whorls densely ornamented with approximately 42 strong axial ribs; intersections of axial ribs and spiral carinas forming prominent elongated nodules at all four carinas, which is strongest and largest at first carina. Whorl face between carinas slightly concave; whorls surface between first and second carinas ornamented with three spiral cords which may be slightly nodular; transition from whorl face to base distinctly angular by fourth carina. Base with up to four distinct spiral cords and weak threads, including one formed by carination after fourth spiral carina. Numerous evenly spaced distinct axial ribs present on whorl face of base; the number of axial ribs decreasing into umbilical area; intersection of basal spiral cords and axial ribs also nodular. Aperture subcircular, four distinct sinuations corresponding to these four carinas; inner lip thin aperture, as wide as high.

Differential diagnosis. This single specimen can be differentiated from other Orbitestella species by the presence of prominent spiral cords, characteristics of nodules, spiral series, axial ribs, and strong carinas at base. This new species is the most similar to Orbitestella praetoreuma Laws, 1939 , from New Zealand figured in Beu & Maxwell (1990) in shell shape and having rows of prominent nodules on upper surface. However, the new species differs from O. praetoreuma in having much denser and more distinct axial ribs. Moreover, the protoconch of the new species is rounded without visible ornamentation, but that of O. praetoreuma is strongly keeled and elaborately sculptured. Orbitestella amphaengensis , new species, also resembles O. bermudezi ( Aguayo & Borro, 1946) as illustrated by Lima et al. (2011) from Brazil. However, O. bermudezi has two peripheral cords, while the new species has four prominent spiral cords, and the former species has denser and finer spiral threads and weaker axial ribs on upper whorl surface, is much smaller, and has a shallower umbilicus. Orbitestella decorata Laseron, 1954 , described from New South Wales has more convex whorls and the keels are not sharp but are broad and low. Orbitestella patagonica Simone & Zelaya, 2004 , from Argentina has much more rounded whorls without keels and ribs, which distinctly differs from O. amphaengensis , new species, in shape and sculpture. Orbitestella ponderi Linse, 2002 , from Argentina has a more protruding peripheral spiral cord and lacks nodules on upper whorl. Orbitestella hinemoa Mestayer, 1919 , from New Zealand is also similar in shape and profile, but O. hinemoa differs in details of ornaments such as the presence of nodules, spiral threads, and axial ribs.

Distribution. This new species is only known from the type locality thus far ( Fig. 1 ).

Figures

Fig. 3. Holotype of Orbitestella amphaengensis, new species (ESKU-2021-I-001). A, apical view; B, early whorl; C, front view; D, protoconch; E, umbilical view; F, close-up of umbilical view.

Fig. 1. Approximate location of type locality of Orbitestella amphaengensis, new species: Amphaeng Sub-district, Ban Phaeo District, Samut Sakorn Province, Thailand.