i9°5-] 



ORTM ANN — AFFINITIES OF CAMBARUS. 103 



this species speaks for its position here. The geographical distribu- 

 tion, however, is entirely opposed to it, and I very strongly suspect 

 that it belongs somewhere else. 



GEOGRAPHICAL DISTRIBUTION OF THE SUBGENUS CAMBARUS. 



Taken as a whole, the subgenus Cambarus occupies a rather con- 

 tinuous area, with a possible interruption in northern Mexico: this 

 gap, however, may be due only to the incompleteness of our 

 knowledge. It covers Mexico, and a large part of the southern, 

 central and eastern United States, but leaves unoccupied the 

 mountainous region of the East ; it is lacking in the larger part of 

 Tennessee, in Kentucky, West Virginia, Pennsylvania, and north- 

 ward. The largest number of species is found in the southeastern 

 states : Mississippi, Alabama, Georgia, and this region represents 

 at present the center of frequency of the subgenus. From here it 

 extends, gradually declining, westward into Texas, northward up 

 the Mississippi valley, becoming quite scarce north of the State of 

 Missouri (only two species), and further it has populated the 

 Atlantic coast plain as far north as New Jersey (only one species 

 north of South Carolina). 



Regarding the single sections, the distribution shows rather 

 peculiar features. The digueti-section is characterized by a marked 

 discontinuity : two species are found in Mexico, one in Cuba, and 

 two in New Mexico, Texas and Kansas. Since I consider this 

 section the most primitive of the genus, this discontinuity is highly 

 interesting, and tends to confirm this view. And further, this 

 peculiar distribution probably indicates the direction of the immi- 

 gration into the United States. The most primitive forms {C. 

 digiteti and mexicanus) are still preserved in the original home of 

 the genus, in Mexico, while two other, somewhat more advanced 

 species ( C. simulans and gailinas) occupy the higher plains lying 

 to the east of the Rocky Mountains in the southwestern United 

 States. These parts are largely formed by Cretaceous deposits, 

 and represent the first land-connection between western and eastern 

 North America after the Upper-Cretaceous separation. It is very 

 significant, that just these parts contain the most primitive forms 

 of the United States, and thus the distribution of the digueti-section 

 clearly indicates this old condition prevailing at the end of the 



