262 MOLLUSCA. 



long tentacula, the eyes being on their external side, and at about the third 

 of their length. 



ToENATELLA., Lam. 

 The shell is elliptical, the spire but slightly salient, the aperture lengthened 

 into a crescent and widened below, and the base of the columella marked 

 by one or two large plica or oblique callosities. 



Pyramidella, Lam. 

 The spire is turreted, the aperture crescent-Hke and wide, and the base of 

 the columella obliquely contorted and marked with sharp spiral plicse. 



Janthina, Lam. 

 The form of the animal separates the Janthinae from all the preceding 

 genera. Their shell, however, is similar to that of the terrestrial Limaces, 

 the columellar margin being also indented, but slightly angular at the ex- 

 ternal edge, and the columella somewhat extended beyond the half oval, 

 which, without this prolongation, would be formed by that edge. 



The animal has no operculum, but the under sui-face of its foot is furnish- 

 ed 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 cylindrical proboscis, terminated by 

 a vertically cleft mouth, and armed with little hooks, has a bifui-cated tenta- 

 culum on each side. 



The common species. Helix janthina, L., has a pretty violet shell, and is 

 very abundant in the Mediterranean. When the animal is touched, it dif- 

 fuses a thick fluid of a deep violet colour that dyes the surrounding water. 



Nerita, Lin. 



The columella of the Neritse being in a straight line, renders the aperture 

 semicircular or semi-elliptical. This aperture is generally large in com- 

 parison with the shell, but is always furnished with an operculum which 

 completely closes it. The spire is almost effaced, and the shell semi-glo- 

 bular. The genus now forms several subdivisions. 



FAMILY II. 



CAPULOIDA. 



Recent researches have convinced us that it is to the Trochoida 

 that we must approximate this family, which contains five genera, 

 four of which are taken from the Patellae. They all have a widely 

 opened, scarcely turbinated shell, with neither operculum, emargi- 



