1902.] AND ANCIENT GEOGRAPHY. 341 



into Antarctica, and consequently into the southernmost parts of 

 America (Chili). Thence it extended, in the beginning of the 

 Tertiary, into Northern Argentina and Southern Brazil (Archiplata). 

 This west-easterly direction of migration from Chili to Brazil is in 

 a certain degree expressed in the present distribution of the genus 

 Paraslacus, and the distribution of the genus ALglea seems to have 

 been formed under similar conditions, although its Antarctic 

 origin does not seem probable. The present southern boundary of 

 Parasiacus is possibly due to the present climatic conditions, it 

 having died out in the south of Chili and Patagonia on account of 

 the unfavorable climate of these parts. 



The fact that the genus does not extend northward into the 

 truly tropical parts of Brazil needs further explanation. We shall 

 return to this later. 



The presence of the genus Parasiacus on both slopes of the Cor- 

 dilleras (even the identical species is found in one case on both 

 sides, and in this respect the genus ALglea agrees with Parasiacus) 

 points to a time when the Cordilleras had not yet attained their 

 present elevation. As v. Ihering has shown, for many groups of 

 animals this chain forms a very sharp barrier, and it does not seem 

 probable that these freshwater Crustaceans are able to cross these 

 high snow and ice-covered mountains. In the case of Parasiacus 

 agassizi a shifting of the continental divide (by the capturing of 

 the headwaters of a stream belonging to the drainage of the oppo- 

 site side) cannot explain its presence on both slopes, since in this 

 region the original divide seems to be intact (the waters of Lake 

 Nahuel Huapi drain to the Atlantic Ocean). Thus also this fact is 

 in favor of an early origin of the distribution, since the elevation 

 of the Cordilleras, although beginning at the end of the Creta- 

 ceous, did not attain its maximum till about the Miocene. 1 



6. CONNECTION OF THE WEST INDIES WITH CENTRAL AND SOUTH 



AMERICA. 



We have seen above that Cambarus cubensis of Cuba finds its 

 most closely allied species in C. mexicanus of Mexico. Similar 

 conditions prevail among the species of Pseudothelphusa from the 

 Greater Antilles, two species {P. americana and terrestris') being 

 also found in Mexico. On the other hand, the six species of the 



1 According to Hatcher, the Miocene Patagonian and Santa Cruzian beds are 

 largely disturbed in the region of the Cordilleras in Southern Patagonia. 



