The subphylum Diasoma (Runnegar and Pojeta, 1974) is a phylogenetic taxon used to show the commonality of origin, ecology, and gross structure of the classes Rostroconchia, Pelecypoda (=Bivalvia), and Scaphopoda. Diasomes have pseudobivalved, bivalved, or univalved shells, which often gape at the anterior and posterior ends. The mouth and anus open at opposite ends of the shell. The head is poorly differentiated or absent, and the radula is present or absent. The stratigraphic range is lowest Cambrian through Holocene.