1902.] AND ANCIENT GEOGRAPHY. 355 



Mexican State of Chiapas, which adjoins Guatemala. As regards 

 Guatemala, we know that here old rocks appear which belong to 

 the system of the Antillean continent (see above, p. 342). Thus we 

 have reason to assume that, while Mexico was covered by the Lower 

 Cretaceous seas which separated North and Central America, this 

 whole region became land at about the middle of the Cretaceous, 

 thus effecting a connection of Western North America with Central 

 America (Guatemala) or with the old Antillean continent. This 

 seems to be also the view of Hill, and he likewise believes that this 

 connection was never subsequently interrupted. 1 



The result of the foregoing discussion is that during the Jurassic, 

 and especially during the Lower Cretaceous. North America formed 

 a unit, which was separated from Asia and which was also circum- 

 scribed by a shore line in the south, being disconnected from 

 Central America. In the middle part of the Cretaceous Mexico 

 was elevated, and this new-formed land connected the western part 

 of North America with the Antillean continent. At about the 

 same time a connection of Western North America with Northeast- 

 ern Asia was established (by way of Bering Sea), and the Mexican 

 Gulf extended northward, separating Western from Eastern North 

 America. 



Thus we have, in the Uppe?' Cretaceous, a strip of land extendi/ig 

 fro?n Northeastern Asia over Bering Sea and over the western side 

 of North America to Mexico and the Antillean continent. Eastern 

 Nofth America was separated from this strip. 



In the beginning of the Tertiary Eastern North Atnerica became 

 reunited to this western section. 



At the end of the Tertiary the Beringian connection with Asia was 

 interrupted (see above, p. 317). 



This would lead us for our Crustaceans to the following conclu- 

 sions: We have seen above (p. 319) that at any time, beginning in 

 the Upper Cretaceous, Potamobius may have invaded the western 

 parts of North America. This is again supported by the preceding 



1 See Hill, 1893, p. 323. Spencer (1897) assumes that there was a reêstab- 

 lishment of the connection of the Atlantic and Pacific Oceans across the Isthmus 

 of Tehuantepec in late Tertiary times. The evidence for it, however, is entirely 

 insufficient. The gravels found on the passes of the isthmus are of no value, 

 since their marine character has not been demonstrated. Compare, also, Hill, 

 1898, p. 262, footnote. 



