24 MOLLUSC A. 



The lohorh of the shell mTLj cither remain in one plane, or incline 

 towards the base of the columella. . 



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

 ing the spire, which is so much the more acute, as the whorls de- 

 scend more rajjidly, and the less they increase in width. These shells 

 with a salient sjjine, are said to be turbinated. 



Wlien, on the contrary, the whorls remain nearly in the same 

 place, and do not envelope each other, the spine is flat, or even con- 

 cave. These shells are said to be di^coidal. 



When the top of each whorl envelopes the preceding ones, the 

 spire is hidden. 



The part thro\igh which the animal appears to come out is named 

 the aperture. 



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

 crawls, it has its shell placed vertically, the columella crosswise on 

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

 opening opposite to the columella. 



AVhen 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 heart 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. This relation is exactly inverted Avith re- 

 spect of the organs of generation. 



The organs of respiration, which are alwaj'^s situated in the last 

 whorl of the shell, receive the ambient element from under its edge, 

 sometimes because the mantle is entirely detached from the body 

 along this edge, and sometimes because it is perforated there by a 

 hole. 



It sometimes happens that the margin of the mantle is prolonged 

 in the form of a canal, in order to allow the animal to seek the am- 

 bient element without exposing its head and foot beyond its shell. In 

 such a case as this, the shell has also in its margin, near the extremity 

 of the columella, opposite that to which the spire inclines, a fissure or 

 canal, for the purpose of lodging that of the mantle. The canal, 

 consequently, in ordinary species, is on the left ; and in the reversed, 

 on the right. 



The animal, however, being very flexible, can vary the direction 

 of the shell, and most commonly Avhen there is a fissure or canal, it 

 directs the latter forwards, which throws the spine behind, the colu- 

 mella to the left, and the opposite margin to the right. It is the 

 contrary in the reversed, for which reason their shell is said to be 

 contorted to the left. 



