Williamia Monterosato 1884

Williamia Monterosato, 1884 Allerya Mörch, 1877 (invalid: Suppressed and placed on the Official Index by Opinion 1410). Brondelia Bourguignat, 1862 (invalid: Suppressed and placed on the Official Index by Opinion 1410)— Rehder 1984a: 83. Scutulum Monterosato 1877: 427 (invalid: junior homonym of Scutulum Tournouer, 1869 [Echinodermata]). Type species Ancylus gussoni Costa, 1829, by monotypy. Williamia Monterosato 1884: 150 (nom. nov. pro Scutulum Monterosato, 1877 non Tournouer, 1869). Type species Ancylus gussoni Costa,1829, by typification of the replacement name— Rehder 1984b: 159; White et al. 2011: 295. Parascutum Cossmann 1890: 883 (invalid: unnecessary nom. nov. pro Scutulum Monterosato, 1877 non Tournouer, 1869) Roya Iredale 1912: 218. Type species Roya kermadecensis Iredale, 1912, by monotypy.— Powell 1934: 155. Taxonomic remarks. Our morphological and molecular studies confirm that Siphonaria and Williamia are closely related. Indeed, their sister group relationship has recently been confirmed by a phylogenomic study of the Panpulmonata (Krug et al. 2022). Morphologically, the patelliform shell of Williamia is distinct from that of Siphonaria. Williamia is distinguished by the combination of the following characteristics: Fragile, externally no ribs or pale axial bands, interior ADM complete circular. By contrast, Siphonaria exhibiting a much greater specific diversity, is quite variable in many shell characteristics, such as geometry, colour, position of apex and ribbing, variable thickness, strongly to unraised external ribs. Most significantly, Siphonaria differs from Williamia by having an internal, horseshoe-shaped ADM. Several Siphonaria exhibit shells that somewhat resemble that of Williamia (e.g., S. oblia, S. lateralis, S. thersites, S. compressa Allanson, 1958 [not examined]). However, these species clearly differ in anatomical characters. The external morphology is similar in both Siphonaria and Williamia. Both groups have a head anterior to posterior shell apex, dual cephalic lobes with central eye spots, mantle, right lateral position of siphon and rectum, pneumostomal lobe (although it is longer in Williamia), and similar sizes of ducts and spermatophore (e.g., S. obliquata, S. sipho to S. radiata.). The radula dentition differs between Siphonaria and Williamia. In Williamia, the central tooth is small and unicuspid, and demarcation of inner and outer lateral teeth is more prominent, inner laterals strongly bicuspid and outer laterals single plates. Hubendick (1946: 8, 18) recognized Williamia as a distinct genus within Siphonariidae distinguished from Siphonaria by a combination of anatomical and shell characters. However, not all differences are confirmed herein for all species. For example, the BC is in front of the adductor muscle [ADM] in Williamia but also in S. camura sp. nov. The shape of the adductor muscle impression in Siphonaria differs from Williamia having an unbroken circular scar, but this is also seen in S. radiata. The BD in Siphonaria runs through or outside this the RAM is correct for most species, but not for S. camura sp. nov. However, it never runs through the RAM in Williamia.