Avicula macroptera. A. lotorium. A. semi-sagitta. A. heteroptera. A. falcata. A. crocea. A. Tarentina. A. Atlantica. A. squamulosa. A. papilionacea. A. costellata. A. physoides. A. virens. _A. macroptera._ The rounded Avicula. Pl. 14, fig. 3. Species oval, oblique, the earlike appendages very developed, especially the superior; one tooth at the hinge. _A. Atlantica._ The Atlantic Avicula. Obliquely curved, yellowish fawn colour, with dark reddish-brown stains; interior pearly. 5. Meleagrina. Two species. The form of the shell is orbicular and equivalve, without the elongated transverse base on the cardinal tooth, and the sloping sides of the opening for the passage of the byssus are perceptible on both valves; these peculiarities distinguish it from the Avicula. Shell sub-equivalve, rotundate, nearly square, externally squamose; the inferior cardinal margin straight, not caudate anteriorly; a sinus at the posterior base of the valves for the passage of the byssus; the left valve being at this place narrow and channelled; hinge linear, without teeth. Meleagrina margaritifera. Meleagrina albina. _M. margaritifera._ The pearl-bearing Meleagrina, sometimes called the mother-of-pearl oyster. Species slightly oblique, somewhat square, pearly, very thick, compressed; undulated and transversely striated, with a series of lamellated longitudinal scales; exterior greenish, interior pearly. This shell is celebrated for its irridescent colours, and is valued for the beautiful and costly pearls it produces. These pearls are formed from a deposition of the substance destined to line the shell upon sand or other bodies, casually or purposely introduced within the mantle of the animal; the shell itself is the mother-of-pearl used for inlaying, or making various elegant trinkets. FAMILY XVIII. PECTINIDES. Seven genera. 1. Pedum. The Shepherd’s Crook. One species. The common name was given to this genus from the resemblance to a French shepherd’s crook. The shell is of a regular form; its lower valve, in which is a sinus for the byssus, is turned up at the edges, and the upper valve falls within it. Shell inequivalve, a little eared; apices unequal, distant, rounded, little evident; hinge without teeth; ligament inserted in an oblique groove, prolonged to the summit, and carried within in a kind of spoonlike cavity. _P. spondyloideum._ The spondylus-shaped Pedum. Pl. 15, fig. 5.