Opisthoporus Benson, 1851

Sang, Do Duc, Chinh, Bui Thi & Nhuong, Do Van, 2020, The land snail genus Opisthoporus Benson in L. Pfeiffer, 1851 (Gastropoda: Caenogastropoda: Cyclophoridae) from Vietnam, with description of a new species, Raffles Bulletin of Zoology 68, pp. 103-111 : 104

Opisthoporus Benson in L. Pfeiffer, 1851

Opisthoporus Benson in L. Pfeiffer, 1851: 8; Benson, 1855: 14; von Martens, 1867: 110; Stoliczka, 1872: 265; Nevill, 1878: 263; von Möllendorff, 1900: 136; Kobelt, 1902: 188, 213; 1913: 834; Gude, 1921: 130; Bartsch, 1932: 323; Egorov, 2009: 16; Low & Tan, 2017: 15.

Tubicyclotus – Habe, 1965: 119.

Siphonocyclus von Möllendorff, 1900: 136 ; Egorov, 2009: 17.

Type species. Cyclostoma taylorianum L. Pfeiffer, 1851 .

Diagnosis. Shell discoidal in shape, with spire that varies from rather high conical to nearly flat, and a wide umbilicus, unicoloured or adorned with irregular axial brownish stripes, usually covered by thin hairy periostracum. Terminal part of the last whorl is separated from the penultimate whorl, or completely attached to the penultimate whorl. Body whorl with breathing tube behind apertural margin. Aperture circular, peristome double or single, with an expanded outer lip. Operculum calcareous, circular, multi-lamellar, nucleus central, with external fringe deeply grooved (H. Adams & A. Adams, 1855; Benson, 1855; Egorov, 2009).

Remarks. The Opisthoporus is superficially similar to the Cyclotus Swainson, 1840 , in general shell form, colouration and pattern, and in the structure of the operculum. However, it can be distinguished from Cyclotus by the presence of a breathing tube, which is a curved hollow horn-like projection that opens into the last whorl behind the operculum (Kobelt, 1913; Bartsch, 1932; Egorov, 2009; Páll-Gergely et al., 2016). The genera Opisthoporus , Pterocyclos Benson, 1832 , Rhiostoma Benson, 1860 , and Pearsonia Kobelt, 1902 , have many similar features (see Sutcharit et al., 2019). However, the structure of the breathing tube varies from notch-like to completely tubular in Pterocyclos (see Sutcharit et al., 2014), whereas it always forms a complete tube in Opisthoporus . Rhiostoma differs from Opisthoporus by its calcareous cup-shaped operculum, the peculiar detached and extended terminal part of last whorl. The operculum of Opisthoporus is calcareous, rather thin, and concave on both sides. Pearsonia has the breathing tube situated far away from the peristome and the outer lip forms a peristomal projection near the suture (see Tumpeesuwan & Tumpeesuwan, 2015).

Currently, the genus Opisthoporus is considered a genus (e.g., Stoliczka, 1872; von Möllendorff, 1900; Bartsch, 1932; van Benthem Jutting, 1960; Bragado et al., 2000; Low & Tan, 2017) or subgenus of Cyclotus (e.g., Nevill, 1878; Kobelt, 1912; Gude, 1921; Egorov, 2009; Inkhavilay et al., 2019; Sutcharit et al., 2019). In this paper, we treat Opisthoporus as a full genus. For other details on the systematics and nomenclature of the Opisthoporus and related genera, see Egorov (2009). Nevertheless, the reliability of currently accepted conchology-based systematics has not been thoroughly examined along other lines of evidence, and further research based on anatomical and radula characteristics or molecular analyses will no doubt be needed to answer questions regarding the systematic positions and affinities of the above genera.