Shell thin, equilateral, sub-triangular, inequivalve; one of the valves larger and more convex than the other, prolonged behind by a sort of heel, sometimes recurved, and pierced by a round hole at its extremity; frequently sloped more or less by a cleft of variable form; the opposite valve smaller, more flat, sometimes formed like a lid, having in the interior a system of support variable in form and complication in every true species; hinge limited, prominent, and formed by two articulating surfaces of one valve placed between corresponding projections of the other; a kind of tendinous ligament issuing from the sloping cleft of the shell, by which it is attached to marine bodies. Terebratula vitrea. T. dilatata. T. rotunda. T. flavescens. T. dentata. T. dorsata. T. pisum. T. globosa. T. sanguinea. T. caput-serpentis. T. truncata. T. psittacea. _T. dorsalis._ The dorsal Terebratula. Pl. 17, fig. 1. The summit of the large valve pierced with a round hole, very circumscribed; grooved longitudinally; valves as if cut sloping in the middle line. _T. globosa._ The globose Terebratula. Species smooth, with the valves rounded at their anterior edge. _T. caput serpentis._ The Serpent’s-head Terebratula. Species grooved, with the summit or heel of the large valve deeply hollowed even to the edge of the articulation; the slope rounded; the valves sub-bilobate by the apparent slope of the anterior edge. 3. Lingula. One species. The valves of this genus are united by means of a tubular, fleshy, or membranous peduncle surrounding the narrow part of them, and of which the base is affixed to marine substances. Shell covered with epidermis, sub-equivalve, equilateral, depressed, elongated, truncated anteriorly, summit middle and posterior, without trace of ligament, but supported at the extremity of a long fibro-gelatinous peduncle, which attaches it vertically to sub-marine bodies; multiple muscular impression. _L. anatina._ The Duck’s-bill Lingula. Pl. 17, fig. 2. Covered with a green, shining epidermis, shaped like a duck’s bill, and having a cylindrical peduncle. CLASS IV. MOLLUSCA. TWENTY-TWO FAMILIES. FAMILY I. PTEROPODA. Six genera Some genera of this family are without a testaceous covering, and are mentioned only for the sake of preserving the family entire. 1. Hyalæa. Venus’s Chariot. Two species. This genus derives its common name from a fancied resemblance to a miniature triumphal car.