5250 MOLLUSCA. 



MELANIA, Lamarck. 



A thicker shell ; the aperture, higher than it is wide, enlarges opposite to the 

 spire; the columella without plicae or umbilicus; length of the spire very 

 various. 



The Melaniae inhabit rivers, but are not found in France ; the animal has 

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

 their length. 



TORNATELLA, Lamarck. 



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, Lamarck. 



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

 the columella obliquely contorted and marked with sharp spiral plicae. 



JANTHINA, Lamarck. 



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 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 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 sur- 

 face of the water. The head, a cylindrical proboscis, terminated by a vertically 

 cleft mouth, and armed with little hooks, has a bifurcated tentaculum on each 

 side. 



The common species, Helix janthina, Lin., 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, Linnaeus. 



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

 semi-circular 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. The 

 genus now forms several subdivisions. 



