Some of the oldest Strophomenidae with bifid cardinal processes are described. It is suggested that at least in some of them, the bifid cardinal processes were formed by partial neoteny. The "chilidal plates" of the Sowerbyellinae are regarded as analogous with the socket plates of the Strophomenidae. Further ontogenetical studies are necessary to tell if the family Strophomenidae is monophyletic.
The double cardinal process in the oldest strophomenids