_Animal._ Depressed, oval, somewhat elongated, inclosed between two lobes of a mantle, slit throughout its anterior half, and having pectinated branchiæ adhering to the internal surface; mouth simple, having on each side a long tentacular appendage ciliated in all its external edge, and rolling itself up spirally in the shell. _Shell._ With an epidermis, subequivalve, equilateral, depressed, elongated, truncated anteriorly; the summit median and posterior; no trace of ligament; a long fibro-gelatinous peduncle fixing the shell vertically to submarine bodies; muscular impression multiple. Inhabits the Indian Ocean. One species. Lingula anatina. CLASS IV. MOLLUSCA. Animals soft, inarticulated, furnished with an anterior head, projecting or salient; most frequently with eyes and tentaculæ, or possessing, at their summit, arms disposed in the form of a coronet: their mouth either short, elongated, or tubular, exsertile, and generally armed with hard parts. Mantle diversified, having its edges free on the sides of the body, or the lobes united, forming a sack, which in part envelopes the animal; gills or respiratory organs various, circulation double, one particular, the other general; heart unilocular, sometimes with the auricles divided, and very distant; no medullary cord along the body, but scattered nerves and ganglions. Twenty-two families. FAMILY I. PTEROPODA. Six genera. Some genera of this family are without a testaceous covering, mentioned only to preserve the family entire. 1. Genus _Hyalæa_. Pl. IX. _Animal._ Body enclosed in a shell, winged before, two opposite wings, somewhat retractile, inserted at the sides of the mouth, head distinct, mouth terminal, placed at the junction of the fins; without eyes. _Shell._ Symmetrical, very thin and transparent, valves unequal, flat above, convex below, open like a cleft anteriorly, summit truncated and tridentated posteriorly. Inhabits the Mediterranean. Two species. Hyalæa tridentata. H. cuspidata. 2. Genus _Clio_. Pl. IX. _Animal._ Body free, naked, more or less elongated, tapering to the rear, head very distinct; provided with six, long, conical, retractile tentaculæ, separated into two groups of three each; mouth altogether terminal and vertical, eyes sessile, rudiment of a foot under the neck. This is a molluscous animal without any testaceous covering, but is here placed as leading to other genera which have shells. Inhabits the Indian seas. Two species. Clio borealis. C. Australis. 3. Genus _Cleodora_. Pl. IX.