Species equivalve, not closing; surface smooth and ribbed within; one valve perfectly white, the other of a brownish or reddish colour.
4. Plagiostoma. Ten fossil species.
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.
5. Plicatula. Five species.
Separated from the Spondylus of Linnæus on account of its distinct structure. The ligament is altogether internal; it is without ears, and the prolonged beak so conspicuous in that genus. The Plicatula has the faculty of affixing itself to another body, so that many are found grouped together in clusters. The valves are strongly plaited within and without, closely interlocking with each other.
Shell solid, adhering, sub-irregular, without ears, inequivalve, pointed at the summit, rounded and plaited behind; hinge with two strong teeth in each valve, with a cavity between them, in which the ligament is internally inserted.
Plicatula ramosa.