GASTEROPODA. 113 



Acera carnosa. — The Fleshy Acera. Cuvier in An- 

 nales du Musee, vol. xvi. p. 10. Plate I. fig. 15, 16. 



TRIBE IT. CALYPTRACEA. 



The branchiae are situated in a dorsal cavity near the 

 neck, and elevated within the cavity, or even projecting 

 beyond it; shell invariably external, and covering the ani- 

 mal. 



Genus U.—A^CYLVS.—Draparnaud. 



Generic Character. — Body creeping, enveloped in the 

 shell ; two thick, sub-cylindrical tentacula, with the eyes 

 situated at their internal base ; foot large, elliptical ; son)e- 

 what narrower than the body. Shell thin, obliquely coni- 

 cal; aperture oval, with a pointed apex, which is inclined 

 backwards; margins simple. 



Ancylus oblongus.— The Oblong Ancylus. Plate XIV. 

 fig. 18. Aperture elongated, vertex turned to one side ; 

 with fine concentric striae; pale horn-colour above, inside 

 bluish. One-third of an inch long. Inhabits the fresh 

 waters of Europe. 



Ge}ius \5.— CREFIDVL A.— Lamarck. 



Generic Character. — Head of the animal anteriorly forked, 

 having two conical or nearly cylindrical tentacula, with the 

 eyes placed at their exterior base ; mouth simple, destitute 

 of jaws, and situated in the bifurcation of the head ; bran- 

 chise with long filaments, and projecting from the large 

 branchial cavity ; the mantle never bordering the shell ; foot 

 minute; anal orifice lateral. Shell ovate, or oblong-ovate, 

 convex on the back, and concave beneath ; spire inclining 

 to one side; aperture with a horizontal partition next the 

 spire, extending to nearly the middle of the shell. 



Crepidula aculeata. — The Prickly Crepidula. Plate 

 XIV. fig. 16. Oval, brown, with acute rough striae; vertex 

 k3 



