Runcinoidea

Runcinoidea
The runcinoideans have long been lumped with the Cephalaspidea for convenience but recently have been shown to be a separate group with their own line of descent from primitive forebears. They are small, flattened, and have an undivided notum that has the appearance of one long continuous headshield the length of the body. Between the overhanging notum and body is a subpallial groove. A minute shell may be present posteriorly, either internally or externally. There is no mantle cavity and the gill, if present, is located posteriorly on the right side.