Value of the Shells of Molltisca. 266 



ceous species, which is not distinguishable by the shell with as 

 much facility as by the soft parts. The examples are very few, 

 indeed, if any exist, in which the species are not recognized 

 by their shells with much greater ease than by the soft parts. 



Of the 20,000 species of shells which are known, the spe- 

 cific value of many is more or less doubtful. It is barely 

 possible that a knowledge of the soft parts will aid materially 

 in determining such species. But this is not probable, since 

 the difficulties arise from the graduation of the specific types 

 into each other ; which graduation is, therefore, likely to 

 pervade the whole animal. 



A practical illustration of the value of the shells appears 

 in the fact that Mr Gray, after examining the soft parts of 

 5000 species, associated Argonauta with Carinaria, notwith- 

 standing the well-known statements of Madame Power. 



The general proposition which w^e have thus endeavoured 

 to illustrate, is only a part of a yet more general fact in or- 

 ganic nature, that, in descending from the more to the less 

 comprehensive groups, the characters which are proper to 

 distinguish the groups are more and more perfectly and uni- 

 formly diff^used 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. 



We have thus far considered the subject practically ; that 

 is, we have considered the utility of the shells of MoUusca 

 in existing systems. A few words should be added on the 

 principles which are involved. 



The principles involved in this subject are threefold ; as 

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

 functional relations, and to its embryonic history. On the 

 latter we shall not offer any remark. 



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

 respond, strictly, with the plumage of birds, the furs of mam- 

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

 vertebrated animals. The correspondence is exact so far as 



