ORDER PECTINIBRANCHIATA. 63 
PIRENA, Lam.,, 
We find not only this little sinus below, but likewise a second 
on the opposite side. (Pir. terebralis, Lam.) 
These two subgenera, as well as the melania, inhabit the 
rivers of Southern Europe and of all hot countries. 
_ There are two genera detached from the volute, which, but 
that they are operculated, and have two tentacula, would 
resemble the auricule. We think they may be placed here. 
AcTZON, Montf. TORNATELLA, Lam., 
Where 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 folds or 
oblique callosities. ‘They must be carefully distinguished 
from the acteons of Oken, which appear to be allied to the 
aplysiz. (Voluta tornatilis and bifasciata, Lin., &c.) And the 
PYRAMIDELLA, Lam., 
Where the spire is turreted, the aperture crescent-like and 
wide, and the base of the columella obliquely contorted and 
marked with sharp spiral plice. (Trochus dolabratus.) 
JANTHINA, Lam. 
The form of the animal separates the Janthine 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 surface of the 
