246 



THE MECHANICAL FUNCTIONS. 



cesses are often exceedingly numerous, and occur 

 at regular intervals, frequently shooting out into 

 various anomalous forms. In many shells of 

 the genus S trombus these spines are of great 

 length, and are arranged round the circumfe- 

 rence of the base, being at first tubular, and 

 afterwards solid, according to the period of 

 growth. This is exemplified in the Pterocera 

 scorpio (Lamarck) of which Fig. 110 shows the 

 early, and Fig. Ill the later period of growth. 



no 



A limit has been assigned by nature to the 

 growth of molluscous animals, and to the shells 

 which they form : and there is a certain epoch 

 of their existence, when considerable changes 

 take place in the disposition of the mantle, 

 and in its powers of secretion. Often we find 

 it suddenly expanding* into a broad surface, 

 adding to the shell what may be termed a large 

 lip. Sometimes no sooner has this been accom- 



