GASTEROPODA. 113 



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

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



TRIBE IV. CALYPTRACEA. 



The braiiclute 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 14.— ANCYLUS.— DrajjarnaMc?. 



Generic Character. — Body creeping, enveloped in the 

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

 situated at their internal base; foot large, elliptical; some- 

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

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

 backwards; margins simple. 



Ancylus ohlongus. — 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. 



Genus 15.— CREPIDUL A.— Z:amarc/e. 



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 ; branr 

 chiifi 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 



