SPIRAL UNIVALVES. 279 



periostraca. It is sometimes very thin and smooth, and 

 • sometimes thick and rough, but in every case it affords an 

 admirable protection to the sliell. 



CHARACTERS OF SHELLS. 



Regarding each piece or valve of a shell as a cone, thus 

 for technical purposes adopting the word in its utmost 

 latitude of signification, as applied to any structure com- 

 mencing at a point and increasing downwards, or even 

 almost in a plane outwards, we shall see that the main cha- 

 racter of the shell will depend upon the direction and rate 

 of increase in which the growth takes place. 



In the great majority of univalve shells the increase takes 

 place obliquely ; that is, each layer is wider, or more ad- 

 vanced, on one side than on the other, and the result is 

 that the cone becomes spiral, and the shell comes under the 

 denomination of 



SPIRAL UNIVALVES. 



These, in which the cone in its course from the apex 

 becomes twisted round an imaginary axis, are susceptible of 

 great variations in the closeness, the obliquity, and the rate 

 of increase in the different coils into which the spiral part 

 is twisted. These coils are called whorls. The inner side, 



