GASTEROPODA PECTINIBRANCHIATA. 365 



The animal 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 Janthina, L.; 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 

 aperrure semicircular or semi-elliptical. This aperture is generally 

 large in comparison with 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 shell; 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. cleBlainville forms his family of the IlEMiCTCLosTOMJE.from this g'emis. 



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

 clxxxix. 



(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, 



