Hypselostoma sichomphuense Tanmuangpak & S. Tumpeesuwan, 2025

Hypselostoma sichomphuense Tanmuangpak & S. Tumpeesuwan sp. nov.

Figs 2 C, D View Figure 2 , 4 A – I View Figure 4 , 5 F – L View Figure 5 , 6 C, D View Figure 6 ; Table 2 View Table 2

Material examined.

Holotype: Thailand • 1 empty shell Khon Kaen Province, Si Chomphu District, limestone wall in mixed-deciduous forest, Phu Pha Kham ( 16°50'22.001"N, 102°03'59.001"E), 21.xi.2021, leg. Tanmuangpak, K. ( NHLRU 032 ) (Fig. 3 G – K View Figure 3 ) GoogleMaps . Paratypes: Thailand • 12 specimens preserved in 70 % ethanol ( NHLRU 033–044 ) (Fig. 2 C, D View Figure 2 ) GoogleMaps ; • two shells ( NHLRU 045–046 ) GoogleMaps ; • four specimens preserved in 70 % ethanol ( NHMSU –00064-B ) GoogleMaps ; • one shell ( NHMSU –00064 ). All paratypes collected from the same location as holotype, leg. Tanmuangpak, K. 11.xii.2021 GoogleMaps .

Measurements.

Holotype: SH = 4.20 mm, SW = 4.02 mm, AH = 2.14 mm, AW = 2.24 mm. Paratypes: ( 19 specimens measured). SH = 3.82–4.29 mm (4.03 ± 0.11 mm), SW = 3.12–4.12 mm (3.88 ± 0.25 mm), AH = 1.84–2.27 mm (2.02 ± 0.12 mm), AW = 1.67–2.24 mm (2.01 ± 0.13 mm) (Fig. 7 View Figure 7 ).

Diagnosis.

This new species differs from Hypselostoma phupaman by the presence of a non-prominent lower keel on the last whorl and the absence of basal and parietal apertural barriers. Aperture has three smooth tooth-like swellings (Fig. 4 A, G – I View Figure 4 ), whereas there are no apertural barriers in H. pongrati sp. nov. (Fig. 3 A, G – I View Figure 3 ). Spire shorter than the spire of H. pongrati sp. nov.

Description.

Shell (Fig. 4 A – I View Figure 4 ) concave-conical, wide-spired, with 4 ½ whorls. Large size for the genus, with shell height 3.82–4.29 mm, shell width 3.12–4.12 mm. The suture is deep. Protoconch pitted, without spiral pattern, with one whorl gradually increasing in size to the teleoconch whorls. The teleoconch has 3 ½ whorls, a very fine spiral surface, with growth lines. The body whorl widest, double keeled (prominent upper and weak lower keel). The peristome is thickened and expanded. Aperture has three smooth, tooth-like swellings, viz. an upper palatal plica, a columellar fold, and a infraparietal plica.

Genital system (Fig. 6 C, D View Figure 6 ). Atrium is shorter than the vagina. Penis is longer than the epiphallus, its anterior portion is a short and gradually bulged tube, and the distal end of its posterior portion is bulged as well. Epiphallus is connected to the distal end of the penis. Epiphallus shorter than vas deferens, its anterior portion slender and cylindrical, its central portion gradually bulges, and its posterior portion is a slender tube, white and glossy. Epiphallic caecum is a short tube, attached to the anterior portion of epiphallus. The penial retractor muscle is shorter than the epiphallus, attached to the distal end of the penis. Vas deferens long, slender, entering epiphallus apically. The vagina and free oviduct are cylindrical, and the vagina is shorter than the free oviduct. The gametolytic sac is a long tube, its bulging anterior portion is surrounded by a thin sheath and connected to vagina, the distal end is a long slender and curved tube. The uterus is large, with a very thin prostate gland attached to it. The hermaphroditic duct is loosely convoluted. The albumen gland is yellowish and large. Dart apparatus absent.

Radula ( n = 3) (Fig. 5 F – L View Figure 5 ). Radula comprises of 154–165 (159 ± 5.56) rows of teeth, each row with 33–35 teeth. Radula formula: (9–10) + 7 + 1 + 7 + (9–10). Central tooth small, unicuspid, and triangular. Lateral teeth bicuspid and asymmetrical, consisting of a large endocone and a smaller ectocone. Seven lateral teeth on each side of the central tooth; the first tooth is largest, and the other teeth are sequentially smaller. Marginal teeth are irregular, unequally tricuspid, with an endocone larger than the ectocone and gradually change to polycuspid outwards (Fig. 5 I – L View Figure 5 ). There are nine or ten marginal teeth on each outer side of the lateral teeth (Fig. 5 F – L View Figure 5 ).

Etymology.

The specific epithet sichomphuense refers to Si Chomphu District, Khon Kaen Province, northeastern Thailand, where this species was discovered.

Animal and ecology.

Living animals have cream-colored bodies, with pale brown head and tentacles and black eyes at the tip of the ocular tentacle. This species was found on the limestone wall covered by mixed-deciduous forest (Fig. 2 C, D View Figure 2 ).

Distribution.

Limestone wall at Phu Kham, Si Chomphu District, Khon Kaen Province.

Remarks.

Hypselostoma sichomphuense sp. nov. is quite different in shell shape from all other Hypselostoma species in Thailand, except for the geographically close H. pongrati sp. nov. The unique characteristics of H. sichomphuense sp. nov. are the three tooth-like swellings in the aperture and the last whorl adnate to the penultimate whorl.