FOUMATION OF SHELLS. 217 



of the Achatina zebra, and of which Fig. 109 shows 

 a longitudinal section, may serve as an example of 



a turbinated shell. The axis of revolution is termed 

 the Columella, and the turns of the spiral are de- 

 nominated whorls. In consequence of the situation 

 of the heart and great blood-vessels relatively to 

 the shell, the left side of the mantle is usually 

 more active than the right side ; so that the lateral 

 turns are made in the contrary direction, that is, 

 towards the right.* There are a few species, how- 

 ever, where, in consequence of the heart being 

 placed on the right side, the turns of the spiral are 

 made to the left. Such shells have been termed 

 sinistral, or reversed shells : but this left-handed 

 convolution seldom occurs among the shells of land 

 or fresh-water mollusca. 



It results from this mode of formation that the 

 apex, both of the simple and of the spiral cone, is 

 the part which was formed the earliest, and which 

 protected the young animal at the moment of its 

 exclusion from the egg. This portion may gene- 

 rally be distinguished by its colour and appearance 



* The terms right and left have reference to the position of the 

 animal when resting on its foot ; the head being of course in front. 

 See Gray, Zoological Journal, i. 207. 



