250 MOLLUSCA. 



There are three other genera in this class, Hyalea, Cleodora, and Pyrgo, 

 A very small fossil shell. 



CLASS III. 

 GASTEROPODA. 



The Gasteropoda constitute a very numerous class of the Mol- 

 lusca, an idea of which is afforded by the Slug. 



They generally crawl upon a fleshy disk, situated under the ab- 

 domen, which sometimes however assumes the shape of a sulcus, or 

 that of a vertical lamina. The back is furnished with a mantle 

 which is more or less extended, takes various forms, and in the 

 greater number of genera, produces a shell. Their head, placed 

 anteriorly, is more or less visible, as it is the more or less involved 

 under the mantle; its tentacula are very small, situated above the 

 mouth and do not surround it, varying in number from two to six; 

 sometimes they are wanting; their function is that of touch, or at 

 most that of smell. The eyes are very small, here adhering to the 

 head, and thereto the base, side, or point of the tentaculum; some- 

 times they are wanting. The position, structure, and nature of their 

 respiratory organs vary, and afford the means of dividing them into 

 several familiesj they never, however, have more than a single aortic 

 heart, that is to say, one placed between the pulmonary vein and 

 the aorta. 



Several are entirely naked; others have merely a concealed shell, 

 but most of them are furnished with one that is large enough to re- 

 ceive and shelter them. 



The shell is formed in the thickness of the mantle. Some of 

 them are symmetrical and consist of a single piece; others are non- 

 symmetrical, which, in those species where they are very concave, 

 and where they continue to grow for a long time, become obliquely 

 spiral. 



If we figure to ourselves an oblique cone, in which other cones, 

 always wider in one direction than in the others, are successively 



