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. P. depressa. P. cristata. P. reniformis. P. Australis. _P. ramosa._ The branched Plicatula. Pl. 15, fig. 2. Oblong, trigonal, very thick; strong longitudinal plaits; exterior brown, with a yellow tinge, with reddish arrow-shaped markings; interior white. 6. Spondylus. The thorny Oyster. Twenty-one species. The valves of this genus greatly resemble those of the common oyster, but have ears, and are covered with long recurved or straight-pointed spines. The lower valve is much larger than the upper, and has foliaceous laminæ, by which it is attached to the other substances. They are found in all seas of hot countries, but particularly in the Indian; they adhere to rocks, coral, &c., oftentimes in large groups. The usual colours are red, purple, white, brown, or orange, several of which are sometimes blended in the same shell. Shell solid, adhering, sub-regular, more or less spined, sub-auriculated, inequivalve; the right or inferior valve fixed, much more excavated than the other, and having behind, at the summit, a triangular face, which enlarges and elongates with age; hinge longitudinal, provided in each valve with two strong teeth entering into corresponding cavities; ligament short, almost medial, partly exterior; one sub-dorsal, muscular impression.