of distinguishing Species and Higher Groups. 199 



Mollusca in that elaborate and excellent paper was designed 

 to include all the modifications that were suggested by an 

 examination of the soft parts of 5,000 species. 



The two Lamarckian orders of Gasteropoda, Gasteropods 

 proper and Trachelipods, each contain both naked and testa- 

 ceous Mollusca, but, with some important exceptions, the 

 shells in each order may be distinguished. Mr. Gray's five 

 orders of this class cannot be distinguished by ordinal charac- 

 ters of the shells. 



In the three orders of Cephalapoda, one is indicated by the 

 shells of Nautilus. The other two orders, including Argo- 

 nautidae, contain each one testaceous genus, with many naked 

 genera ; but these testaceous genera are widely different. 



If we modify Mr. Gray's system by dividing the Brachio- 

 poda into two orders, and by regarding Argonauta as the shell 

 of an Octopod, we have (including the Tunicata) seventeen 

 orders, of which two are naked, ten are wholly testaceous, and 

 five are partly testaceous. But of the fifteen which are more 

 or less testaceous, only five are easily separated by ordinal 

 characters in the shells, and the others are distinguished with 

 more or less difficulty. 



In descending to the families, we find the value of the 

 shells much greater than in the orders. Here I will limit my 

 remarks to the families in Mr. Gray's list of genera, both to 

 avoid unnecessary detail, and because they are constituted 

 with more regard to the soft parts than those of any other 

 classification. 



In the class Cephalopoda, the two testaceous genera (Argo- 

 nauta being excluded from the class,) constitute as many 

 families, which are, therefore, amply indicated by their 

 shells. 



In the first order of Gasteropoda, Pectinibranchiata, Mr. 

 Gray makes five families, of which three, Strombidae, Volu- 

 tidaB, and Cyprgeadae, are easily distinguished by their shells ; 



