GASTEROPODA PECTIN'IBRAXCHXATA. 61 



they are operculated and have but two tentacula, would resemble the 

 Auriculse, that we think may come here, viz. 



AcT.EON, Muntf* — ToRXATELLA, Lam. 



Where the shell is elliptical, the spire but slightly silient, the aper- 

 ture lengthened into a crescent and widened below, and the base of 

 the columella marked by one or two large plicre or oblique callosi- 

 ties f; and the 



Pyramidella, La!7i. 



Where the spire is turreted, the aperture crescent-like and wide, 

 and the base of the columella obliqely contorted and marked with 

 sharp spiral plicae];. 



JaNTIIINA, Z(7W.§ 



The form of the animal separates the Janthinje from all the preced- 

 ing genera. Their shell, however, is similar to that of the terres- 

 trial Limaces, the colvunellar margin being also indented, but slightly 

 angular at the external edge, and the columella somewhat extended 

 beyond the half-oval, which, without this prolongation, would be 

 formed by that edge. 



The animal has no opercuhuu, but the under surface of its foot is 

 furnished with a vesicular organ resembling a bubble of foam, but 

 composed of a solid substance, Avhich prevents it from crawling, but 

 allows it to float on the surface of the water. The head, a cylindri- 

 cal proboscis, terminated by a vertically cleft mouth, and armed with 

 little hooks, has a bifurcated tentaculum on each side. 



The common species, Helix Janthina, L,: List., 572, 24, has 

 a pretty violet shell, and is very abundant in the Mediterranean. 

 AVhen the animal is touched, it dift'uses a thick fluid of a deep 

 violet colour that dyes the surrounding water. 



Nerita, Lin. \\ 



The columella of the Nerita? being in a straight line, renders the 

 aperture semicircular or semi-eUiptical. This aperture is generally 

 large in comparison with the shell, but is always furnished with an 

 opercidum which completely closes it. The spire is almost effaced, 

 and the shell semi-globular. 



* Which must be carefully distinguished from the Acfceons of Oken that appear to 

 be allied to the Aplt/sia. 



t Vohtta tonuitilis, and bifasdcda, L. Martini, II, xHii, 442, 443 ; — V. nCcafa, 

 and r. solidida, lb., 440, 441; — V. fummea, lb., 439; V. ftava, lb. 444 ;— T^ 

 pusiUa, lb. 446. 



+ Trochus dolabratus, L. Chemn., V, clxvii, 1C63, 1064; — Btdiiius (erdellum, 

 Brug., List., 844, 72. 



§ This genus forms the family of the Oxystomje, Blainv. 



II M. de Blainville forms his family of the Hemicyclostom.t-, from this genus. 



