GASTEROPODA. 325 



are non-symmetricalj 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 placed, it will be easily seen that the convolution 

 of the whole takes place on the side which enlarges the least. 



This part, on which the cone is rolled, is termed the colu- 

 mella; it is sometimes solid, and sometimes hollow. When 

 hollow, its aperture is called the umbilicus. 



The whorls of the shell may either remain in one plane, or 

 incline towards the base of the columella. 



In this last case the preceding whorls rise above each other, 

 forming the spire, which is so much the more acute, as the 

 whorls descend more rapidly, and the less they increase in 

 width. These shells with a salient spine are said to be tur- 

 binated. 



When, on the contrary, the whorls remain nearly in the 

 same place, and do not envelope each other, the spine is flat, 

 or even concave. These shells are said to be discoidal. 



When the top of each whorl envelopes the preceding ones, 

 the spire is hidden. '^ 



The part through which the animal appears to come out is 

 named the aperture. 



When the whorls remain nearly in the same plane, while the 

 animal crawls, its shell is vertical, the columella crosswise on 

 the hind part of its back, and its head passes under the edge 

 of the opening opposite to the columella. 



When the spire is salient, it inclines from the right side in 

 almost every species; in a very few only does it project from 

 the left when they are in motion; these are said to be reversed. 



It is observed that the head is always on the side opposite 

 to that to which the spire is directed. Thus it is usually on 

 the left, and in the reversed on the right. The case is reversed 

 with respect to the organs of generation. 



The organs of respiration, which are always situated in the 

 last whorl of the shell, receive the ambient element from under 



