of distinguishing Species aud Higher Groups. 197 



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

 ment of 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 ve y ^arge 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 

 labor 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, bnt irregularly,) until we 

 arrive at the species, which may be amply distinguished 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 Gasteropoda, 

 some are naked and others are not, and some have shells^ 

 which can be distinguished from those of the class of Articu- 

 lata only when we descend to the characters proper to genera 

 and species. Of the Pteropods also, which some regard 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) agree only in characters which are 

 common to those of the Gasteropoda, although easily distin- 

 guished in the genera. But if, with Mr. Gray, we regard 

 Argonauta as the shell of a Gasteropod, similar to Carinaria, 

 No. 10—2. 



