Only known as fossils; probably introduced here by Lamarck to serve as a
connecting link for the genera Lima, Pecten, Spondylus, and Podopsis.
Shell rather thick, regular, free, sub-equivalve, sub-auriculated; the
two valves almost equally convex, both provided with a distinct summit,
recurved in the middle of a level surface, with a great triangular slope
in the middle; the cardinal base transverse, straight; hinge without
teeth; a conical cardinal pit situated below the beak, partly internal,
opening outward, and receiving the ligament.
_P. spinosa._ The thorny Plagiostoma.
Subarcuated, the umbo of one shell higher than that of the other, with
longitudinal ribs and remote concentric rings.