T he lower Cretaceous strata of the Neuquén Basin of Argentina are richly fossiliferous and characterized by diverse, primarily normal marine, molluscan-dominated faunas containing many interesting taxa. Among these, in the Lower Hauterivian part of the Agrio Formation at Cerro Mesa (Fig. 1), are diverse, coarsely ornate trigoniid bivalves of the genus SteinmanellaCrickmay, 1930, and a new genus and species of ornate Mytilidae (Nodomytilus trigonimimus). The new taxon appears to be convergent in its surface sculpture on the Austral Gondwanian trigoniid Steinmanella transitoria (Steinmann, 1881). This may represent a rare case of...

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