Value of the Shells of Mollusca. 261 



The study of shells is important, on account of the facili- 

 ties which they furnish for learning the extent of the depart- 

 ment of the Mollusca. Of the 15,000 or 20,000 species which 

 have been described, the soft parts of only a small minority 

 have been described, and of a very large majority, the shells 

 only have been seen by naturalists. I only allude to the value 

 of this subject to geology, and to the utility of a division of 

 labour in the study of the details of natural history. 



The proposition which I intend to illustrate is this : that 

 the value of the shells of Mollusca, as a source of distinctive 

 characters, commencing at zero for the entire department, 

 increases as we descend through the less comprehensive 

 groups (not, of course, in a uniform ratio, but irregularly), 

 until we arrive at the species, which may be amply distin- 

 guished by the shells only. 



That the shells are of no value for the purpose of distin- 

 guishing the department of Mollusca, is evident from the 

 multitude of naked Mollusca. 



In the first subdivision into classes, they begin to be of 

 some value. The class Brachiopoda is distinguished by two 

 valves, one dorsal and one ventral ; the Conchifera always 

 by two lateral valves ; and the Tunicata, which otherwise 

 much resemble the Conchifera, are naked. But of the Gas- 

 tropoda, some are naked and others are not, and some have 

 shells, which can be distinguished from those of the class of 

 Articulata only when we descend to the characters proper to 

 genera and species. Of the Pteropods, also, which some re- 

 gard as entitled to the rank of a class, many are naked, but 

 the rest have peculiar shells. While a large majority of the 

 existing Cephalopoda are naked, the shells of the testaceous 

 species (if we include Argonauta) agi^ee only in characters 

 which are common to those of the Gastropoda, although 

 easily distinguished in the genera. But if, with Mr Gray, 

 we regard Argonauta as the shell of a Gastropod, similar to 

 Carinaria, the testaceous species have a well-marked charac- 

 teristic in being polythalamous. 



It is very obvious, therefore, that the shells begin to be of 

 some value in the first subdivision into classes, since of the six 



VOL. LI. NO. CII. — OCTOBER 1851. 



