GASTEROPODA PULMONEA. 253 



' The Aquatic Pulmonea have but two tentacula; they are con- 

 tinually compelled to rise to the surface for the purpose of breath- 

 ing, so that they cannot inhabit very deep water; they are usually 

 found in fresh water or salt ponds, or at least in the vicinity of the 

 sea coast and of the mouths of rivers. Some of them have no shell, 

 such as those of the genus Onchidium. 



The aquatic Pulmonea, with complete shells, were also placed 

 by Linnasus in his genera Helix, Bulla and Voluta, from which 

 it has been found necessary to separate them. 



In the first were comprised the two following genera, where we 

 find the internal edge of the aperture crescent-shaped, as in Helix. 



Planoebis, Brug. 

 The Planorbes are distinguished from the Helices by the slight increase of 

 the whorls of their shell, the convolutions of which are nearly in one plane, 

 and because the aperture is wider than it is high. It contains an animal 

 with long, thin filiform tentacula, at the inner base of which are the eyes, 

 and from the margin of whose mantle exudes a quantity of a red fluid, which 

 is not, however, its blood. In stagnant waters. The 



LiMNiEus, Lam. 

 Has, like a Bulimus, an oblong spire and the aperture higher than it is 

 wide; but the margin, like that of a Succinea, is not reflected, and there is 

 a longitudinal fold in the columella, which runs obliquely into the cavity. 

 The shell is tliin ; the animal has two compressed, broad, triangular tentacula, 

 near the base of whose inner edge are the eyes. 



They inhabit stagnant waters in great numbers; they also abound with 

 the Planorbes in certain layers of marl or calcareous strata, which they evi- 

 dently prove were deposited in fresh water. 



Physa, Drap. 



The Physx have a shell very similar to that of a Lymnxus, but without the 

 fold in the columella and reflected edge, and very thin. When the 

 animal swims or crawls, it covers its shell with the two notched lobes of its 

 mantle, and has two long, slender and pointed tentacula, on the greatly en- 

 larged internal base of which are the eyes. They inhabit springs, &c. 



Auricula, Lam. 

 Differing from all the preceding aquatic Pulmonea in the columella, which 

 is maiked with wide and oblique flutes. Their shell is oval or oblong, the 

 aperture elevated as in Bulimus, and the margin tumid. 



CoNovuLus, Lam. 

 Projecting folds in the columella, as in the Aui-iculae, but the margin of the 



