BULL^A. MOLLUSCA. 63 



B. physis, Lin. Shell rounded, very smooth, pellucid, white, with 

 waved transverse brown stripes ; spire obtuse. 1 inch long. In- 

 habits Indian seas. Klein, Ost. pi. 5, fig. 98. 



Gen. 9. BULL^A, Lam. Bulla, Lin. 



Shell oval-oblong, slightly convex above, divided transversely 

 into two parts ; lateral lobes of the foot thickened and reflect- 

 ed upwards ; head indistinct ; no tentacula ; branchiae dorsal ; 

 shell concealed in the mantle, thin, partially involute on one 

 side, and without columella or spire ; aperture very wide. 



B. aperta, Lam. Shell somewhat rounded, pellucid, slightly striated 

 transversely, and the aperture very large. ^ inch in diameter. In- 

 habits coasts of Europe. B. Penn. Brit. Zoo/, iv. pi. 73. 



Gen. 10. ACERA, Lam. 



Body oval-convex, divided transversely above into two parts, 

 and winged inferiorly by the lateral dilatations of the foot ; 

 head indistinct ; branchiae on the back, much behind, and 

 covered by the mantle ; no shell. 



A. carnosa, Lam. About an inch and a-half long. Inhabits Me- 

 diterranean sea An. Mus. xvi. pi. 1, fig. 15, 16. 



FAMILY IV. CALYPTRACEA. 



Branchiae placed in a particular cavity on the back, near the 

 neck, and projecting beyond or included in the cavity ; shell 

 always exterior. 



Gen. 11. ANCYLUS, Lam. Patella, Lin. 

 Body covered by the shell ; two compressed tentacula, slightly 

 truncated, with the eyes at their internal base ; foot short, 

 elliptical, a little narrower than the body ; shell thin, in the 

 form of an oblique cone, with the summit pointed, inclined 

 backwards, and the margin simple. 



This genus is formed of fluviatile shells, which Linnaeus included in the genus 

 Patella. They are very small animals, and are found on the margins of lakes and 

 slow running waters, crawling on aquatic plants. 



A. lacustris, Lam. Shell semiovate, membranaceous, with the sum- 

 mit pointed and reflected, and the base oval. 3 lines long. Fresh 

 waters in Europe. B. Brown's Illust. pi. 36, fig. 3, 5, 7- 



Gen. 12. CREPIDTJLA, Lam. Patella, Lin. 

 Animal with the head forked anteriorly ; two conical tentacula, 

 with the eyes at their exterior base ; mouth simple, without 

 jaws, placed in the bifurcation of the head ; branchiae tufted, 

 projecting out of the branchial cavity ; mantle never border- 

 ing the shell ; foot very small ; anus lateral ; shell oval or 

 oblong, with the back almost always convex, concave below, 

 and the spire inclined ; aperture partly closed by a horizon- 

 tal plate. 



The shells of this genus are found on the shores attached to rocks. 



