198 Value of the Shells of Mollusca for the yurpose 



the testaceous species have a well marked characteristic 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 classes tvi'o are wholly testaceous, and may be distin- 

 guished, not by the shells alone, but by their position on the 

 soft parts ; and three are partly testaceous, of which two may 

 be distinguished by their shells so far as they are testaceous. 



In the orders^ we find their vahie not much increased. 

 The two orders of Brachiopoda are distinguished by a single 

 character of the shells, namely, the presence or absence of 

 articulation between the valves. This is not the hash of their 

 ordinal classification, which would be artificial if founded on 

 one character, but it is an index of differences which charac- 

 terize the entire animals. 



The two Lamarckian orders of Conchifera are separated 

 on account of a difference in their soft parts, namely, the 

 number (whether one or two) of the adductor muscles, which 

 character is clearly indicated in the shells of all the families 

 except in the TridacnidaB,Svhere the approximation of the two 

 muscles presents the appearance of a single impression. In 

 the Mytilidse, which have several adductor muscles, some of 

 which are inconspicuous, the defect in the ordinal character 

 of the soft parts is itself indicated in the shells. In Mr. G-ray's 

 list of the Genera of Recent Mollusca", five orders of Conchi- 

 fera is enumerated. I have not seen them characterized, but 

 it is obvious from their genera that ordinal characters cannot 

 be found in the shells. The two orders of Pteropoda in Mr. 

 Grray's list, as in several other authors, are indicated by the 

 presence or absence of a shell, although the classification of 



* Since tliis article was read before the Association, we have seen, with great 

 pleasure, the complete solution of the problem of the Argonauta in the " Voyage of 

 the Samarang." Arthm* Adams, Esq., Surgeon and Natui-alist of the Samarang, 

 found that the paper Nautih are the Ggg cases of an Octopod, carried about by the 

 female. 



