On the validity of the Permian bivalve family Pachydomidae Fischer 1887

Australian bivalve genera referred to the Family Pachydomidae Fischer (1887) and Newell (1956) are compared in details of dentition and ligament with genera placed in the Edmondiidae King (1850). It is found that the hinges of Myonia Dana (1847) and Pachymyonia Dun (1932) are edentulous like those of Edmondia de Koninck (1844). Megadesmus Sowerby (1838) ( = Pachydomus Morris 1845) has a rudimentary tooth in the right valve, but its anterior retractor connected to the adductor by the protractor resembles the musculature of Myonia and Pachymyonia, and the internal ligament is in between that of Edmondia unioniformis and E. josepha, so that the differences are scarcely of subfamilial and probably not of familial rank. Other genera referred to the Pachydomidae differ more. Pyramus, Notomya, and Globicarina have a well‐defined tooth in the right valve and a discrete anterior retractor, and Astartila has a tooth in each valve, and a discrete anterior retractor. Astartila appears to be close in its dentition to the Early Carboniferous genus Scaldia de Ryckholdt (1852). Thus, whilst the present study is incomplete because of the uncertain nature of the hinge and musculature of some edmondiid genera of the European Carboniferous, it appears that the Family Pachydomidae is polyphyletic, with several generic lineages, of which at least two can be traced back to Early Carboniferous members of the Family Edmondiidae.