GASTEROPODA PECTINIBKANCHIATA. 365 



The animnl has no operculum, but the under surface of its foot is 

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

 composed of a solid substance, which 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 Janihma, h.; List., 572, 24, has 



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



When the animal is touched, it diffuses a thick fluid of a deep 



violet colour that dyes the surrounding water. 



Nerita, Lin.(l) 



The columella of the Neritae being in a straight line, renders the 

 aperture semicircular or semi-elliptical. This aperture is generally 

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

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

 and the shell semi-globular. 



Natica, Lam. 



Neritse with an umbilicated shelly the animal of the species known 

 has a large foot, simple tentacula with the eyes at their base, and a 

 horny operculum(2). 



Nerita, Lam. — Peloronta, Oken. 



The umbilicus wanting; shell thick, columella dentated, and oper- 

 culum stony; the eyes of the animal on pedicles by the side of the 

 tentacula, and a moderate foot(3). The 



Velata, Montf., 



Where the side of the columella is covered with a calcareous, 

 thick and convex layer(4), is distinguished from it, but perhaps 

 without any good reason; also the 



Neritina, Lam., 

 Where the shell has no umbilicus and is thin, with a horny oper- 



(1) M. de Blainville forms his family of the IiEMiOTci.osTOM;B, from this g-enus. 



(2) For the species see the first div. of Gm. and Chemn., "V, pi. clxxxvi — 

 clxxxlx. 



(.3) For the species see the third div. of Gm. and Chemn., V, pi. clxxxx — 

 clxxxxiii, and Sowerby, Gen. of Sh., No. XV. 



(4) Nerita perversa, Gm., a large fossil species; Chemn., IX, cxiv, 975, 976. 



