XV 



number of extinct families admitted is larger than is generally 

 recognized, especially in the class of Cephalopods. The views 

 of M. Barrande have been implicitly accepted in the arrange- 

 ment of the families of Tetrabranchiates, save as to the value of 

 the groups. M. Barrande has designated the Mollusca as a 

 class, the Cephalopoda as an order of that class, and has sub- 

 divided the latter into three families, each comprising a greater 

 or less number of genera. The standard of value applied by 

 that learned naturalist is in each case, but especially in the appre- 

 ciation of the major groups, very different from that almost uni- 

 versally current, and as the more comprehensive groups are 

 here retained with the higher rank generally accredited to them, 

 the genera are also raised to a more elevated rank : the views of 

 M. Barrande concerning the range of his genera being provision- 

 ally accepted, they are each one raised to family rank, and 

 although the author is disposed to dissent from the positions 

 assumed by M. Barrande in respect to the affinities and extent 

 or relative value of certain of his genera, his knowledge of those 

 forms is so vastly inferior to that naturalist's, that he has not 

 ventured in any case to depart from him, even when he would 

 have simply accepted the views of others, for none have had such 

 opportunities for study, or made such good use of them, as he. 

 As the expediency of the extension of family rank to some of 

 the forms may be questioned, it may be remarked that the ten- 

 dency of some naturalists seems to be to even subdivide still 

 more minutely, Prof. Agassiz and Prof. Hyatt, for example, dif- 

 ferentiating the genus Ammonites of most authors into a number 

 of families, and separating ordinally the "Ammonoids" from the 

 Nautilidae. 



In addition to the numerous extinct types of the Cephalopods, 

 there are undoubtedly many among the Gasteropods and Con- 

 chifers that are entitled to family rank ; but in view of the 

 inability of the author to study many of them, and of our igno- 

 rance of their relations, it has been deemed inadvisable to name 

 thenii 



SYNONYMY. 



In order to make known the extent of the families adopted, 

 as well as to direct students to reliable sources of information, 

 reference has been made to a specific authority for each family. 



