356 ÓRTM ANN— DISTRIBUTION OF DECAPODS [April 3, 



considerations, in so far as it is confirmed that Potamobius cannot 

 have been present in North America during the Lower Cretaceous, 

 otherwise the remarkable restriction to the west would be inexplic- 

 able. But the genus must have immigrated during the Upper Cre- 

 taceous, since fossil remains of Potamobiidce 1 are known from the 

 Eocene of North America. This latter fact, therefore, narrows 

 down the time of immigration to more definite limits (those of the 

 Upper Cretaceous), and at the same time explains its restriction to 

 the west. During this period the western parts of the country 

 were separated from the eastern by sea. At the same time there 

 was a possibility for the crayfishes to reach Mexico, and it is easily 

 understood that Potamobius then sent a branch southward, which 

 subsequently developed on the Mexican plateau into Cambarus. 

 After this, in the beginning of the Tertiary, Cambarus had a chance 

 to migrate by way of Texas into Eastern North America, where it 

 reaches its culmination in the present time. 



The morphological differentiation of Cambarus from Potamobius 

 probably took place in the beginning of the Tertiary, after the 

 ranges of these genera had become separated. This separation is 

 apparently due to a climatic change in the region between Mexico 

 and central California, where desert conditions developed. This 

 desert climate is not so pronounced on the eastern side of the con- 

 tinent, near the Atlantic coast in Texas, and, consequently, the area 

 of Cambarus is not here interrupted between Mexico and the 

 United States. A subsequent connection of the ranges of Potamo- 

 bius and Cambarus in the interior ol North America Cm the region 

 of the Rocky Mountains and the plains adjacent to their eastern 

 slope) was impossible on account of the topographic and climatic 

 barrier existing there in Tertiary times, which has been mentioned 

 above (p. 354). The Rocky Mountains themselves and the arid 

 regions are not favorable for the freshwater crayfishes. Thus the 

 areas of both genera remained separated, and only in one case a 

 species {P. gambeli) has crossed the continental divide in the 

 region of the Yellowstone National Park. This, however, is very 

 likely due to the capturing of streams that originally belonged to 

 the Pacific slope by the Yellowstone river. 



b. Central America. 



The tectonic unity of the old Archaic and Paleozoic rocks known 



1 Cambarus priinceviis of Packard, from the Eocene of Western Wyoming, 

 which is, however, according to Faxon (1885, p. 155), rather a Potamobius. 



