3. Genus Navicella. Pl. [X].

Animal. Oval, not spiral, foot elliptical, very large, with a thin subpapillary edge, advanced rather anteriorly, without marginal furrow, but attached on each side to the visceral mass in all its posterior portion so as to form a sort of cavity open transversely behind; head very broad and semilunate; tentacula conical, contractile and distant; eyes subpedunculated at the external root of the tentacula; mouth large and longitudinal.

Shell. Fluviatile, elliptical, or oblong, convex above, summit straight, depressed to the margin, concave beneath; no columella; the columellar edge replaced by a sharp partition, covering part of the aperture; a sinus at its left extremity; muscular impression horse-shoe shaped, open in front and interrupted behind; thin, calcareous operculum, with a subulate, lateral tooth adhering to the posterior margin. Inhabits rivers in the Isle of France. Four species.

4. Genera Natica. Pl. [XI].

Animal. Oval, subspiral; foot profoundly and transversely bilobed anteriorly, and having a horny or calcareous operculum; head provided with long setaceous tentacula, flat, and auriculated at the base; eyes sessile at the external side of the root of the tentacula; mouth armed with a labial tooth.

Shell. Marine, distinguished from the Nerita by having no teeth, and having an umbilicus modified by a callosity; very smooth and glossy; no epidermis; spire evident, but low; columellar edge not toothed; right edge without teeth; operculum calcareous and smooth, semi-spiral, with concentrical ribs, fitting a groove on the columella. Inhabits the W. Indian Ocean and American seas. Thirty-six species.

FAMILY XII.
Janthinea. One genus.