GASTEROPOD.V PECTINIBRANCHIAT A. 61 
tliey are operciilatecl and have but two tentacula, would resemble the 
Auriculae, that we think may come here, viz. 
Act.eon, Montf* * * § — Torxatella, Lam. 
Where the shell is elliptical, the spire but slightly salient, the aper- 
ture lengthened into a crescent and widened below, and the base of 
the columella marked by one or two large plicae or oblique callosi- 
ties f; and the 
Py KAMIDPJLf.A, Lam. 
Wliere the spire is turrcted, the aperture crescent-like and wide, 
and the base of the columella oblicjcly contorted and marked with 
sharp spiral plicaej. 
Jantiiina, L«/7?.§ 
The form of the animal separates the Janthinro from all the preced- 
ing genera. Their shell, however, is similar to that of the terres- 
trial 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 tliat 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 s\ibstance, which prevents it from crawling, but 
allows it to float on the surface of the water. Tiie 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 Jant/iina, L,: List., 572, 24, has 
a pretty violet shell, and is very abundant in the Mediterranean. 
W^hen the animal is touched, it diffuses a tliick fluid of a dee]) 
violet colour that dyes the surrounding water. 
Nerita, Lm. II 
The columella of the Neritae being in a straight line, renders the 
aperture 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. 
* Which must be carefully distinguished from the Actccons of Oken that appear to 
be allied to the Aphisia. 
t Vohda tornatilis, and bifasciafa, L. Martini, II, xliii, 442,443; — V. sidcata, 
and r. solidula, Ib., 440, 441; — V. fummeu, Ib., 439; V. fiaru, Ib. 444 ; — V. 
piutilla, Ib. 446. 
+ Ti-ochiis dolahmtusy L. Cliemn., V, clxvii, li 6J, 1064 ilnrt aas /crdellum, 
Brug., List., 844, 72. 
§ This genius forms the family of the OxYSTOMiE, Blainv. 
II iSI.de Blainville forms his family of the IIemicvclostom.i', from this genus. 
