282 OKTMANN — DISTRIBUTION OF DECAPODS [April 3, 



Nova Scotia is excluded. Isolated from the continuous Mexican 

 and United States ranges is the Island of Cuba, where C. cubensis 

 has been found. 



It is hard to say where the centre of the whole genus is situated. 

 Judging from the number of species represented in the different 

 parts, it seems to be more in the East than in the West, but for the 

 rest the genus is pretty evenly distributed in the Southeastern 

 States, in the region of the Alleghanies and the central basin, and 

 decreases markedly only in a westerly direction, disappearing 

 before it reaches the foothills of the Rocky Mountains. In the 

 Southwest, in Texas and New Mexico, the genus is less abundant, 

 and in northern Mexico it is found only near Parras, in the State of 

 Coahuila ; but then again it becomes more abundant in the central 

 part of Mexico. Whether this apparent scarcity in northern 

 Mexico and Texas corresponds to the actual conditions, or whether 

 it is due to defective knowledge of these parts, cannot be decided. 

 One result, however, is very evident : the genus is preeminently 

 characteristic of the central and eastern parts of the United States, 

 there attaining its highest development as regards the number of 

 species. 



Now, what is the origin of this distribution of Cambarus ? Did 

 this genus originate in these parts, or whence did it come, and 

 which are its ancestors ? 



In order to answer the first question, we learn much by recalling 

 to our mind the distribution of the single groups as stated above. 

 We have seen that the centre of the first group is in the Southeast ; 

 the range of the second group — although somewhat discontinuous — 

 centres in the Southwest. The third group has evidently its centre 

 in the mountainous regions of the Allegheny system, the fourth 

 group in the central basin and the fifth in Mexico. 



The second and fifth groups are strongly represented in the 

 Southwest, the first group has distinct relation to these parts, the 

 fourth group only a few isolated stations, while the third group is 

 entirely wanting there. 1 



1 Faxon (1885, P- x 7^) expresses this in the following way: in the South 

 (Mexico, Cuba, Gulf States and Atlantic States south of North Carolina) species 

 of the first, second and fifth groups prevail, while comparatively few species of 

 the third and fourth groups are present ; in the North (Atlantic States north of 

 South Carolina, Central States and Canada) species of the third and fourth 

 groups prevail, while only a few species of the first and second advance into the 

 northern provinces. 



