_Animal._ Head anteriorly forked, having two conical tentacula, with the eyes placed at their exterior base; mouth simple, destitute of jaws, and situated in the bifurcation of the head; branchiæ with tufts and projecting from the branchial cavity; the mantle never bordering the shell; foot minute, orifice lateral. _Shell._ Ovate, or oblong; the back almost always convex, concave beneath; the spire very much inclined towards the margin; the aperture partly closed by a horizontal lamina. Inhabits the American seas. Thirteen species. Crepidula extinctorum. C. aculeata. C. lævigata. C. uguiformis. C. dilata. C. Peruviana. C. gigas. · · · · · C. fornicata.* C. depressa.* C. glauca.* C. plana.* C. intorta.* C. convexa.* 7. Genus _Ancylus_. Pl. X. _Animal._ Body creeping, enveloped in the shell; two compressed subtruncated tentacular, with eyes situated at their internal base; foot short, elliptical; somewhat narrower than the body. _Shell._ Thin, obliquely conical; aperture oval, with a pointed apex, which very much inclines backwards; margins simple. This is a fresh water shell, found in the lakes of Europe and rivers of America. Five species. Ancylus lacustris. A. fluviatilis. A. spinarosæ. · · · · · A. rivularis. A. filosus. FAMILY IV. BULLACEA. Three genera. 1. Genus _Acera_. _Animal._ Body ovate, convex, transversely divided above into two parts; the foot with dilations in the form of wings below; head indistinct; branchiæ situated on the back, greatly behind, and covered by a mantle destitute of a shell. Inhabits the Mediterranean. One species. Acera carnosa. 2. Genus _Bullæa_. Pl. X. _Animal._ Body ovate, somewhat convex above, and divided into two parts transversely; lateral lobes of the foot very thick; head indistinct, and without tentacula; branchiæ placed on the back. _Shell._ Concealed in the mantle, very thin, rolled and spiral on one side; without a columella and spire; aperture very large and wide, dilated at the upper part. Inhabits the British seas. Two species. Bullæa aperta. B. striata. 3. Genus _Bulla_. Pl. X. _Animal._ Body oblong-ovate, slight convexity; divided into two portions transverse above, mantle slightly folded posteriorly; visible tentacula; branchiæ dorsal and covered, opening only on the right side. _Shell._ Univalve, ovate globular, convolute, no columella, spire not projecting but visible, aperture the whole length of the shell, external margin sharp and smooth. Inhabits the British and American seas. Nineteen species. Bulla physis. B. naucum. B. hydatis. B. rugosa. B. fragilis. B. aplustre. B. ampulla. B. lignaria. B. fasciata. B. striata. B. Wyatii. B. solida. B. cornea. B. lactea. · · · · ·