1002. J AND ANCIENT GEOGRAPHY. 351 



and this restricted land-bridge going across the middle part of 

 the Atlantic existed probably during the rest of the Cretaceous 

 time and was not destroyed until the beginning of the Tertiary, a 

 chain of islands remaining as late as the Oligocene. 



This means, with respect to our freshwater crabs, that their age 

 goes back at least to the Upper Cretaceous. During this period the 

 last remnant of the continental connection between Africa and 

 Guiana still existed, and the absence of Potamonidoz in South 

 America south of the Amazonas valley further substantiates this 

 assumption, that these crabs did not reach South America prior to 

 the Upper Cretaceous, when the main part of Brazil also took part 

 in this old continental connection. Aside from this fact, we have 

 the consideration that it is not very likely that the age of the fresh- 

 water crabs goes far back in Cretaceous times. Although we have 

 no definite information as to the latter point, we may say, from a 

 morphological standpoint, that the Potamonidce represent a pecu- 

 liarly specialized side branch of primitive Cyclometopa. Cyclome- 

 topa existed in the beginning of the Cretaceous, but were rare. 

 Thus an Upper Cretaceous age of the Potajnonidoz is admissible. 



The subsequent fate of the Potamonidoz in South and Central 

 America, after they immigrated (or originated) in these parts in the 

 later Cretaceous, will be discussed in the next chapter. 



S. THE MUTUAL RELATIONS OF NORTH, CENTRAL AND SOUTH 



AMERICA. 



Aside from the peculiarities in the distribution of the freshwater 

 Decapods of America, discussed above, there are several other 

 features which need explanation. They are the following (see pp. 



2 95> 2 9 6 > 3°9): 



r. The remarkable restriction of the genus Potamobius to the 

 western parts of North America, while Cambarus is found in the 

 east and south (Mexico). 



2. The southern limit of the range of Cambarus. 



3. The distribution of the Potamocarcinince over the West Indies, 

 Central America and the northern parts of South America ; their 

 presence in the mountains of Ecuador and Peru and their absence 

 in Brazil south of the Amazonas. 



4. The peculiar shape of the areas of Parastacus and AZglea, 

 which are almost identical, and extend, in the subtropical and tern- 



