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.