202 



Vahte of the Shells of Mollusc a for the jno'pose 



difficulties arise from the graduation of the specific types into 

 each other, which graduation is, therefore, likely to pervade 

 the whole animal. 



The general proposition which we have thus endeavored to 

 illustrate, is only a part of a yet more general fact in organic 

 nature — that in descending from the more to the less compre- 

 hensive groups, the characters which are proper to distinguish 

 the groups are more and more perfectly and uniformly diffused 

 through the parts of the entire being. We have been informed, 

 on the best authority, that the most kindred species of fishes 

 may be distinguished by their scales only. Species of birds 

 may be distinguished |'by their feathers, or by their eggs. If 

 we descend to individuals, nothing but observation is wanting 

 to detect their peculiarities in each part of the entire being. 



"W* have thus far considered the subject practically ; that 

 is, we have considered the utility of the shells of Mollusca in 

 existing systems. A few words should be added on the prin- 

 ciples which are involved. 



The principles involved in this subject are three-fold ; as 

 they belong to the structural relations of the shell, to its func- 

 tional relations, and to its embryonic history. On the latter 

 we shall not offer any remarks. 



The shells of Mollusca, having their origin in the skin, 

 correspond, strictly, with the plumage of birds, the furs of 

 mammals, and the scales of fishes ; in general, to the covering 

 of vertebrated animals. The correspondence is exact so far as 

 it relates to the peculiarities of color. But the details of form 

 are more perfectly represented by the skins of Vertebrata than 

 by the shells of Mollusca. Yet most of the details of form in 

 the soft parts, which are not moulded in the shells, are indi- 

 cated by constantly associated characters. 



In their functional relations, shells are skeletons. They 

 serve for the attachment of muscles and the support of the 

 soft parts. On this ground, therefore, we may claim for them 



