1902.] AND ANCIENT GEOGRAPHY. 285 



noticeable from its centre in the middle Mississippi basin toward 

 the East, North and West. 



Thus we are to recognize the fact that the different groups, 

 chiefly the first, third and fourth, express in their distribution a 

 regular, continuous advance in a northeasterly direction. Toward 

 the North and East is continuity, which represents a more recent 

 stage in distribution, while in the opposite direction, toward South- 

 west, we observe discontinuity, which characterizes generally a 

 more ancient stage. In the second group we have a very remark- 

 able discontinuity, and this group is a comparatively primitive one, 

 and the fifth group, which is also primitive in some degree, is 

 chiefly found in the Southwest. 



All the foregoing considerations tend to justify our conclusion 

 that the migration of the genus Cambarus into the United States 

 started in the Southwest, on the Mexican plateau, and advanced in 

 a northeasterly direction. 



Taking up now the second point to be considered, the question 

 of the origin and the ancestral forms of the genus Cambarus, we 

 shall be satisfied — for the present — with the opinion of Faxon 

 (1885, p. 16), which is also that of the present writer, that this 

 genus is the most highly specialized within the family Potamobiidce, 

 a corollary of which is that it must have originated from forms of a 

 lower type, which probably corresponded to the genus Potamobius ; 

 in fact, it is easy to imagine that Cambarus is derived directly from 

 Potamobius by the suppression of the single posterior pleuro- 

 branchia and the high specialization of the copulatory organs. 

 However, before entering into a more detailed discussion of the 

 relation of Cambarus and Potamobius, we shall give a sketch of the 

 chorology of the latter genus. 



Genus Potajnobius} 



It is advisable here to go more into detail, since, on the one 

 hand, a synopsis of the more recent publications in this group is 

 desirable, and since, on the other, the number of species in this 

 genus is comparatively small and our knowledge of them excellent. 

 The genus is divided into two subgenera : Potamobius sens, strict. 

 Ortm. {Astacus sens, strict. Fax.) and Cambaroides Fax. 



1 The following facts have not been put together since Faxon's review (1885). 

 I shall use here chiefly the revision of this group which I have prepared for the 

 " Thierreich." 



