An exceptionally well-preserved silicified bivalve from the Upper Permian of Texas is described and assigned to a new genus and species, Cassiavellia galtarae, placed in the family Bakevelliidae. The species represents one of the earliest and best characterized unequivocal occurrences of the multivincular ligament in the superfamily Pterioidea. The silicified material provides a wealth of information on the morphology of inadequately known Paleozoic pterioideans, including hitherto undescribed aspects of the larval shell, auricular sulcus, muscle scars, and dental ontogeny. The discovery of the condyle-fossa complex on the anteroventral shell margin, a feature previously undescibed in Bivalvia, raises the question of the homology and taxonomic significance of the problematic subumbonal ridge-like structures in Pterioidea. In life, C. galtarae was probably an epifaunal right-pleurothetic bivalve, byssally attached to hard or raised flexible substrata. In addition to C. galtarae, another new species, C. nadkevnae, is placed in Cassiavellia.