Abstract
One of the chief characteristics in many members of the bivalve super-family Pholadomyacea is the presence of an inner depression along the hinge. It is shown that the depression varied in development in upper Paleozoic members referable to the Edmondiidae and subsequently became shorter in Mesozoic members of the Pholadomyidae, to culminate in the short pit seen in living Pholadomya. Runnegar (1966, 1967) on the other hand denied the presence of a pit in either Pholadomya or upper Paleozoic allies, except Edmondia. Particular attention is paid to features of musculature and pallial sinus and hinge in Upper Paleozoic genera from western Europe. It is shown that one subfamily, Pachydominae Fischer, was not restricted to a particularly unusual and largely Permian group endemic to Gondwana, as claimed by Runnegar, but was widespread with close allies in the Lower Carboniferous.
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