Xll 



FAMILIES. 



The author has applied the views of those who consider those 

 groups, above the rank of genera, combined by numerous com- 

 mon characters, and distinguished from neighboring groups 

 by greater or more abrupt differences than those existing within 

 the limits of such common associations, to be entitled to 

 family rank. In Articulates, Vertebrates, and Radiates, such 

 groups are often recognizable externally by a similarity of 

 form which is dependent on more or less decided modifi- 

 cations of structure, or the relations between different parts. 

 Very often, however and especially in the Batrachians such 

 indications fail, and in the Mollusks there are many families that 

 do not differ from each other in form ; and, on the other hand, 

 others exhibit a very considerable difference of form among their 

 own representatives. Accepting the views as to the application 

 of the term family to groups as adopted by the students of Mam- 

 mals, we must apply them as we best can to the Mollusks, and 

 of course we must be prepared for considerable diversity of views 

 in the application, dependent on the personality of the observer, 

 his acquaintance with the groups, and the path by which he has 

 approached the study. 



Tery many, and probably most of the families now adopted, 

 require revision based on more extensive materials than have 

 yet been available to any one investigator. If any are to be 

 especially pointed out in this connection, those of the orders 

 of Cephalopods, and among the Gasteropods, the TurMnellidde* 

 Pupinidae, and the sub-divisions of the disintegrated Helicidse, 

 Melaniidee, Cerithacea, and Trochacea, may be indicated. But, 

 because their affinities are doubtful, they have been for the present 

 retained, for it is believed that the evils resulting from hetero- 

 geneous combinations (not definable by diagnosis) is greater than 

 those resulting from refinement of analysis. 



The acquaintance of the author with the Polyzoa being ex- 



1 The Turbtnellidce are retained as distinct on the authority of a very 

 distinguished naturalist, who has kindly informed me that they are 

 "Stromboidce." I have not ventured to separate them, however, farther 

 from the Cynodontidce till more is known. 



