Pterocyclos Benson, 1832

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

Genus Pterocyclos Benson, 1832

Pterocyclos Benson, 1832: 11. Wenz, 1938: 461. Egorov 2009: 35. Sutcharit et al. 2014: 331-333.

Pterocyclus Agassiz, 1848: 908 [unjustified emendation]. Kobelt 1902: 160, 161. Kobelt 1911: 719. Gude 1921: 97. Egorov 2009: 26, 27.

Types species.

Pterocyclos rupestris Benson, 1832 by monotypy.

Diagnosis.

Shell medium to large, discoidal, conical to nearly flattened. Shell colour uniformly yellowish to with brownish stripes; periostracum thin or thickened and hairy. Terminal part of last whorl usually attached to penultimate whorl or sometimes separated with short detached whorl. Peristome circular and double or sometime boundary hardly visible; lip thickened and expanded. Breathing device on upper junction of peristome; outer peristome slightly to broadly expanded; inner peristome inconspicuous or with a shallow incision. Umbilicus widely opened. Operculum calcareous, low cup-shaped to concave, inside covered with corneous layer, outside with calcareous anticlockwise multispiral with elevated lamella, and lateral straight.

Remarks.

The genus comprises a hundred nominal species ( MolluscaBase 2022) and is widely distributed from South to Southeast Asia. The comprehensive revision of the genus has never been implemented other than in the classical work by Kobelt (1902, 1911). Seven species have been reported so far from the Malay Peninsula (see Sutcharit et al. 2014). The genus Pterocyclos can be distinguished from the Rhiostoma by having an expanded outer lip with wing shape and forming an incomplete tube, inner lip with deep and wide incision, inside of operculum with corneous layer and outside of operculum calcified with wide to close lamellae. In contrast, the Rhiostoma tend to have a detached whorl, breathing device of various types, outer lip not expanded, and thick calcareous cup-shaped operculum without corneous layer inside and with wide and elevated lamella outside (Table 2 , Fig. 5 ).

Figures

Figure 5. Comparison of breathing device and operculum characteristics among the five genera that closely resemble Rhiostoma. The specimens and opercula represented herein are the type specimen or topotype of the respective type species of each genus; Pterocyclos: P. rupestris Benson, 1832 (lectotype UMZC I. 102070, no locality data), Spiraculum: S. hispidum Pearson, 1833 (specimen NHMUK 1906.4.4.72, from Khasi, Teria Ghat), Rhiostoma: R. haughtoni Benson, 1860 (syntype UMZC I. 103445, from Moulmein), Cyclotus: C. variegatus Pearson, 1833 (specimen NHMUK 1896.6.7.34 - 35, from Palawan), Opisthoporus: Cyclostoma taylorianum Pfeiffer, 1851 = junior synonym of Pterocyclos biciliatum Mousson, 1849 (specimen NHMUK 20210200 for shell, from Basan, Sarawak) and Pterocyclos tener Menke, 1856 (NHMUK ex. Cuming collection for operculum, from Cochin China), and Ptychopoma: Cyclophorus chinensis Moellendorff, 1875 (lectotype SMF 39415, from Lushan bei Kiukiang, China). Red arrows indicate the expansion of outer lip at upper junction of the peristome. Blue arrows indicate breathing tubes (apertural tube in Rhiostoma and sutural tubes in Spiraculum and Opisthoporus).