Reinterpretations are made of North American Pennsylvanian rostroconchs based on shell form, shell structure, and surface ornamentation. The new family Pseudobigaleaidae, new genera Baiosoma, Hadropipetta, Exalloschema, Oxyprora, and Apotocardium are erected, and the new species Arceodomus angustus and Apotocardium plautum are recognized. Study of growth increments on Apotocardium lanterna (Branson) indicate annual and monthly growth rates and life span.
Thin sections studied provide insights into shell layer structure and thickness and variation in shell form. Characteristics of the protoconch and its relationship to body shell layers are illustrated. Epifaunal elements consisting of worm tubes, inarticulate brachiopods, acrothoracic barnacles, bryozoans, corals, and algae are present, some suggest attachment to the host rostroconch in life position.