316 FUNCTIONS AND INSTINCTS. 



world, that God has created such wonderful 

 beings. Specimens have been found of the 

 enormous diameter of six feet. Though the 

 sculpture of many of these great cephalopods 

 gives reason to think that they may be inter- 

 mediate between the argonaut and nautilus, 

 yet the convolutions and external form of their 

 conchs gives them no small resemblance to a 

 genus of snails, 1 the species of which are often 

 found in fresh waters, except that in this the 

 shell is more concave on one side than the other. 

 The genus Spirula, the animal of which appears 

 also to be a Cephalopod, 2 seems to exhibit the 

 first tendency to this form. 



Amidst all this variety of Molluscous animals, 

 exhibiting such diversity in their structure and 

 organization, in their habits, food, modes of life, 

 and stations, one great object seems attained 

 by their creation especially, the production of 

 calcareous matter. Even the shells of terres- 

 trial testaceans, if we consider the vast numbers 

 that every year perish, must add in no trifling 

 degree to the quantity of that matter on the 

 earth, and probably make up for the continual 

 waste or employment of it, so as to maintain 

 the necessary equilibrium ; but in the ocean, the 

 quantity added to that produced by corallines, 



1 Planorbis. * 2 PLATE VII. FIG. 2. 



