Epidiceras Dechaseaux 1952

GenusEpidicerasDechaseaux, 1952 [= Eodiceras Pchelintsev, 1959 = Megadiceras Pchelintsev, 1959 = Mesodiceras Pchelintsev, 1959 = Paradiceras Pchelintsev, 1959] Type species. Diceras sinistra Deshayes, 1824, by original designation. Diagnosis. Small to very large Epidiceratidae (maximum commissural dimensions vary from 25 to more than 150 mm) characterised by sub-equivalve to moderately inequivalve shells with sub-circular, slightly high-ovate or D-shaped commissures. Outside of shell well-rounded to distinctly posteriorly carinate. Anterior and posterior adductor muscles inserting on distinct myophoral ledges, which extend along the anterior and posterior walls of the shell cavity, below the hinge plate. Remarks. The name Epidiceras was proposed as a new genus in a general account of hippuritid evolution by Douvillé (1935). Most subsequent workers (including the first author) overlooked that Douvillé (1935) had not selected a type species, and his new generic name was thus unavailable. The name Epidiceras was made available by Dechaseaux (1952), who selected Diceras sinistra Deshayes, 1824 as the type species. Epidiceras has a long external ligament and generally well-pronounced, free myophoral ledges, which are extending along the anterior and posterior walls of the shell cavity below the hinge plate. In contrast, both adductor muscle scars are placed on downward extensions of the hinge plate in Plesiodiceras and Heterodiceras. As a result, the shell cavities of Plesiodiceras and Heterodiceras are much smaller and their internal moulds much slenderer than those of Epidiceras, which additionally bear the incisions left by the dissolved myophoral ledges. Furthermore, Epidiceras typically has sub-equivalve shells; the free right valve is often only moderately smaller than the attached left valve. In Plesiodiceras, the right valve is much smaller, almost flat, resembling a gastropod operculum. In Heterodiceras, the right valve is often larger than in Plesiodiceras, but still far from sub-equal to the left valve. Gemmellaro had proposed a new subgenus, Pseudodiceras, in the programme of the first congress of the Società italiana per il progresso delle scienze in 1875 (Gemmellaro 1875: 28; original ‘ programme’ not available to the authors). This subgenus was supposed to contain the carinate forms Diceras carinatum (here assigned to Epidiceras) and D. affine (here referred to Heterodiceras) from Sicily, which Gemmellaro thought to be distinguished by the auricular shape of their posterior cardinal tooth (3 b) in the right valve. However, after consulting Bayle’s (1873) monograph, which displayed specimens with cardinal teeth of intermediate shapes, Gemmellaro refrained from formally proposing the new name. As a result, Pseudodiceras was established in synonymy (Gemmellaro 1875: 28, 29) and is thus a nomen nudum. The name is also not available from Cox (in Dechaseaux et al. 1969: N 778), who designated Diceras carinatum Gemmellaro, 1875 as the type species, but did not use Pseudodiceras as a valid name. Pchelintsev (1959) established four new genera, Eodiceras (type species: Diceras ursicinum Thurmann, 1853), Megadiceras (type species: Diceras beyrichi var. porrecta Boehm, 1883), Mesodiceras (type species: Mesodiceras enissalense Pchelintsev, 1959) and Paradiceras (type species: Chama speciosa Münster in Goldfuss, 1837), which were regarded as junior subjective synonyms of Epidiceras by Skelton (1999, 2013 a), based on their attachment by the left valve and the presence of myophoral ledges; this view is followed here. Pchelintsev’s (1959) generic diagnoses are based on minor differences in outline shape of the commissure, shell size, shape of the hinge teeth, and the size and elevation of the posterior myophores, which we largely regard as significant at species level, but within the intrageneric variability of Epidiceras.