CLASS GASTEROPODA. 21 
PyReGo, 
A very small fossil shell, discovered by M. Defrance, globular, 
very thin, divided by a transverse cleft, very narrow, except 
in front, where it widens a little. 
THIRD CLASS OF THE MOLLUSCA. 
THE GASTEROPODA 
Constitute a very numerous class of mollusca, of which some 
notion may be formed from the slug and snail. 
They generally crawl upon a fleshy disk placed under the 
belly, but which sometimes assumes the form of a furrow, or 
that of a vertical lamina. The back is furnished with a man- 
tle, which extends more or less, assumes various figures, and 
produces a shell in the greater number of genera. ‘Their head, 
placed in front, is shown more or less according to the degree 
in which it is engaged under the mantle. It has only small 
tentacula, which are above the mouth, and do not surround it ; 
their number varies from two to six, and they are sometimes 
wanting; their use is only for touch, or at most for smell. 
The eyes are very small, sometimes adherent to the head, 
sometimes to the base, or the side, or the point of the tenta- 
cula; they are also sometimes wanting. ‘The position, the 
structure, and the nature of the respiratory organs, vary, and 
give rise to their division into several families. But they 
have only an aortic heart, that is placed between the pulmon- 
ary vein and the aorta. 
The position of the apertures through which the organs of 
