3. Genus Lingula. Pl. [IX].

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.

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].