Spondylus gædaropus. S. Americanus. S. arachnoides. S. candidus. S. multilamellatus. S. coccineus. S. crassisquama. S. spathuliferus. S. ducalis. S. longitudinalis. S. costatus. S. variegatus. S. longispineus. S. regius. S. avicularis. S. microlepos. S. croceus. S. aurantius. S. radians. S. zonalis. S. violacescens. _S. gædaropus._ The thorny red Spondylus. Pl. 15, fig. 1. Upper valve red, under one white, with longitudinal striæ or ribs; rough granulations, and somewhat tongue-shaped; rather short truncated spines. _S. longispineus._ The long-spined Spondylus. Thickly spined, longitudinally sulcated and ribbed; alternate spines arcuated and tongue-shaped; valves of a reddish colour; umbones orange. 7. Podopsis. Two fossil species. Only introduced here to fill up the family and keep up the chain of connexion. It approximates the genus Gryphea. Shell rather thick, sub-regular, symmetrical, equilateral, inequivalve, adhering by the extremity of the shorter valve; the other terminated by a summit pointed, a little recurved and medial; articulation very angular, by means of two very distant condyles. FAMILY XIX. OSTRACEA. Six genera. 1. Gryphæa. One species. This genus resembles the Ostrea, with which it was formerly classed, but from which it is distinguished by the peculiar character of the lower valve. It is very deep and carinated, with a summit terminating in a long spirally recurved beak, slightly turned to one side; the edge sharp and angular. It is seldom, if ever, attached to other bodies. Shell more finely lamellated than that of the oyster, free or slightly adherent, sub-equilateral, very inequivalve; the lower valve very concave, with a summit more or less recurved like a hook; the upper valve much smaller, and formed like a lid; hinge without teeth; ligament inserted in an oblong arched cavity; one single muscular impression on each valve. _G. angulata._ The angulated Gryphæa. Pl. 16, fig. 2. Oblong ovate, with three long longitudinal carinated ribs below; summit of the inferior valve is subvolute. This is a rare shell. 2. Ostrea. The Oyster. Forty-eight species.