Elongated, wedge-shaped, covered with regular elevated jagged or scalloped ribs, elegantly disposed; three dorsal accessory pieces; muscular impression almost medial. _P. crispata._ The curled Pholas. Somewhat oval, truncated behind, and as if divided into two parts by an oblique furrow from the summit to the base; anterior part reticulated, the other parts plain; muscular impression marginal. _P. clavata._ The clubbed Pholas. Short, wedge-form, little gaping, with many accessory pieces. 2. Gastrochæna. Three species. Always without accessory pieces, and, therefore, easily distinguished from the Pholas. Shell equivalve, somewhat wedge-shaped, with a very large, oval, oblique, anterior opening between the valves; the posterior extremity nearly close; hinge linear, marginal, without teeth; two distant muscular impressions; sometimes with a kind of tube or calcareous general envelope. Gastrochæna cuneiformis. G. mytiloides. G. modiolina. _G. modiolina._ The Modioliform Gastrochæna. Oval, thin, brittle, gaping at the side; light reddish brown, with a bluish white interior. _G. cuneiformis._ The wedgelike Gastrochæna. Pl. 3, fig. 1. Species with a smooth shell, and without distinct tube. (Represented as imbedded in wood.) FAMILY III. SOLENIDES. Four genera. 1. Solen. Eighteen species. There are many species belonging to this genus differing considerably in form and appearance. Its name is derived from a Greek word signifying a pipe or tube. It is a bivalve whose breadth sometimes exceeds its length; some species have a resemblance to the sheath of a razor or a knife handle; others are curved like the scabbard of a cimeter. The Solen is found in the sand of the seashore, which it sometimes penetrates to the depth of one or two feet. Most of the species are covered with an epidermis, which renders their colours more or less obscure. In general they present but little beauty, though some are of a bright pink colour, and some are beautifully and delicately radiated with purple and white. The principal characteristic of this genus is the hinge, which generally has one subulate tooth, though sometimes two or three. Shell equivalve, extremely inequilateral, transversely elongated, open at both ends; the apices very small, and entirely at the commencement of the dorsal line; one or two teeth in the hinge; ligament external; two distant muscular impressions; the anterior one very long and narrow, the posterior one sub-angular.