Rhiostoma platymorpha Tongkerd & Tumpeesuwan, 2023

Tongkerd, Piyoros, Tumpeesuwan, Sakboworn, Inkhavilay, Khamla, Prasankok, Pongpun, Jeratthitikul, Ekgachai, Panha, Somsak & Sutcharit, Chirasak, 2023, Systematic revision of the snorkel snail genus Rhiostoma Benson, 1860 (Gastropoda, Caenogastropoda, Cyclophoridae) with descriptions of new species, ZooKeys 1142, pp. 1-144 : 1

20. Rhiostoma platymorpha Tongkerd & Tumpeesuwan sp. nov.

Figs 34 , 41 , 42F

Rhiostoma sp. 1- Tumpeesuwan 2001: 59-61, figs 4.19-4.21 (in part).

Type material.

Holotype CUMZ 4498/1 (cW 19.0 mm, cH 8.4 mm, dL 7.2 mm; Fig. 41A ). Paratypes CUMZ 3976 (22 shells), CUMZ 4349 (1 shell), CUMZ 4436 (4 adults + 1 juvenile), CUMZ 4498/2 (6 shells; Fig. 41C, D ), CUMZ 4731 (1 adult + 1 juvenile), CUMZ 4763 (4 shells; Figs 41B , 42F ), CUMZ 4801 (48 shells), CUMZ 4809 (74 shells), CUMZ 4892 (10 adults + 1 juveniles), CUMZ 10196 (3 shells), CUMZ 10198 (12 adults + 2 juveniles), CUMZ 10199 (1 shell), NHMUK 20220445 (5 shells), and SMF 368680 (5 shells). All paratypes are from the type locality.

Type locality.

Tham Muang On, Mae On District, Chiang Mai Province, Thailand (18°47'10.6"N, 99°14'17.1"E). Limestone hill with dry evergreen forest. The locality is surrounded by a timber plantation and farms for annual crops.

Other material examined.

Thailand: Tham Boa Tong, Mae Tang, Chiang Mai: CUMZ 4437, 4783 (Fig. 41E, F ), 10197.

Diagnosis.

Shell small, thin, and flattened; detached whorl shorter than apertural width. Shell colour with brownish zigzag patterns on both sides. Breathing device with incomplete tube shape (rarely short tubular) and attached to preceding whorl.

Differential diagnosis.

This new species can be distinguished from R. strubelli by having a flattened shell, short detached whorl, and brownish zigzag colour pattern, usually on both the dorsal and ventral sides of the shell. In contrast, R. strubelli has a sub-discoidal to discoidal shell shape, and brownish zigzag pattern only on the dorsal side.

Description.

Shell. Shell small, cW 17.2-20.2 mm, cH 8.8-11.3 mm, thin, and flattened to sub-discoidal shape; detached-whorl length 2.0-6.0 mm. Apex acute; spire nearly flattened to slightly elevated. Whorls 4 to 5, convex, increasing regularly; suture wide and shallow; last whorl rounded and slender. Shell surface with irregular growth lines. Periostracum corneous and transparent. Shell colour with dark to brownish zigzag pattern, and ventral side with paler pattern; narrow dark brown spiral band on periphery. Detached whorl shorter than apertural width and descending. Peristome circular and double; lip slightly thickened and not expanded. Breathing device with incomplete tube shape and its tip usually attached to preceding whorl; outer lip forming a long and nearly closed tube; inner lip with deep incision. Umbilicus widely opened and deep. Operculum calcareous, low cup-shaped, and multispiral (Fig. 41 ).

Radula. Teeth arrangement and morphology are similar to those of R. asiphon . Central tooth with large central cusp and two lateral cusps on each side. Lateral teeth composed of four cusps; central cusp large with pointed tip; two inner cusps with pointed tips; one outer cusp very small. Marginal teeth each composed of three pointed cusps (Fig. 42F ).

Etymology.

The subspecific name platymorpha comes from two Latin words platy, which means flat, and morpha meaning form. It refers to the prominent flattened shell shape of this species.

Distribution.

This new species is known from the type locality, limestone hills in Chiang Mai Province.

Remarks.

The specimens from Tham Buatong, Chaing Mai (Fig. 41E, F ), tend to differ from the type specimen in having sub-discoidal shells. However, the brownish zigzag colour pattern appears on the dorsal and ventral shell surface, and an incomplete tube-shaped breathing device suggests they are closely related to this species. That being noted, a few specimens with monochrome whitish shells also occur in this population; this is probably an albinistic shell (Fig. 41F ).

Figures

Figure 34. Distribution map of Rhiostoma asiphon - species group.

Figure 41. Shell of Rhiostoma platymorpha sp. nov. A holotype CUMZ 4498 / 1 from Tham Muang On, Mae On, Chiang Mai B paratype CUMZ 4763 from type locality C, D paratypes CUMZ 4498 / 2 from type locality E, F specimens CUMZ 4783 from Tham Bua Tong, Mae Tang, Chiang Mai (with F albinistic shell form).

Figure 42. Radula morphology A Rhiostoma asiphon CUMZ 4826 from Angthong Islands, Surat Thani B Rhiostoma strubelli specimen CUMZ 10200 from Vieng Phuka, Luang Namtha, Laos C Rhiostoma breviocollar sp. nov. paratypes CUMZ 4491 from type locality D Rhiostoma anceyi sp. nov. paratype CUMZ 4494 / 2 from type locality E Rhiostoma furfurosum sp. nov. paratype CUMZ 4705 / 2 from type locality F Rhiostoma platymorpha sp. nov. paratype CUMZ 4763 from type locality.