Potamilus capax (Green, 1832)
Other common names: Pocketbook, grandmaw.
Key characters: A rounded, greatly inflated shell, thin to moderately thick, S-shaped hinge line, tan or light brown, rayless, and shiny.
Description: Shell round to somewhat oblong, greatly inflated, and thin (young) to moderately thick (adults). Anterior and posterior ends rounded. .Umbos greatly inflated, elevated, and turned inward. Beak sculpture of a few faint ridges, visible only in young shells. Small posterior wing present in young mussels. Surface usually smooth and very shiny. Periostracum rayless, yellow, yellowish tan, or olive, becoming dark brown in older individuals. Length to 5 inches (12.7 cm).
Pseudocardinal teeth thin, compressed, and elevated; two in each valve. Lateral teeth thin and greatly curved; two in the left valve, one in the right. Hinge line S-shaped. Beak cavity very deep. Nacre white, sometimes tinged with pink or salmon.
Habitat: Large rivers in slow-flowing water in mud or sand.
Status: Federally Endangered.