10 



POPULAR COXCHOLOGY. 



Patella vulgata. 



which is pushed out, it is evident that the conformation 

 of every shell will depend upon the manner in which the 

 mollusc protrudes its mantle, and that it will be spiral, 

 cup-shaped, or otherwise, according to circumstances. 

 This can be best explained by a few references ; thus, the 

 simplest form of shell is that of the Patella or Limpet, 

 which has a very small por- 

 tion ^represented at A in the 

 figure) formed at the period 

 of its birth ; as it increases 

 in size, and finds this in- 

 adequate to protect its body, 

 it expands the border of its 

 mantle, and, by degrees, en- 

 larges its shell all round, until it presents the appearance 

 of A b ; in like manner, at different periods, as its growth 

 requires, it enlarges to C, d 5 and E, at which time, having 

 attained its full size, it ceases to require any farther en- 

 largement, and the shell is then completed or mature. 



This presents the most simple method of growth, as in 

 the Limpet the mantle is extended equally at every part 

 of the circumference of the original shell, and the size 

 merely is increased, still preserving the same shape ; in 

 the greater number of univalve shells, how- 

 ever, the method of enlargement is differ- 

 ent, the mantle being more active on one 

 side of the aperture than on the other : 

 this gives rise to a turbinated or spiral 

 shell, in which each successive portion of 

 the spire, Avhich forms an entire circle, is 

 called a whorl. A shell ( Scalaria pretiosa) 

 of this nature is here shown. The new 

 formations can generally easily be traced, 

 and the ridges represented in this species 

 consist of a thickened lip, which, at certain periods of its 



Scalaria jirctiosa. 



