3Õ4 ORTMÀNN — DISTRIBUTION OF DECAPODS [Aprils, 



cally, the Upper Cretaceous sea barrier being replaced by a barrier- 

 formed by the Rocky Mountains. 1 



Looking now toward Mexico and its continuation southward, we 

 shall refer in the first place to the papers of Hill (1893 and 1898). 

 The history of Mexico in Pre-Cretaceous times is very obscure. 

 Possibly it was covered by sea, as is also assumed by Neumayr, in 

 the Jurassic, at least in part (Hill, 1898). But it seems to be well 

 established that in the Lower Cretaceous (Hill, 1893) almost all of 

 Mexico was submerged from the Atlantic to the Pacific side. This 

 Lower Cretaceous sea was limited on the north by the southern 

 coast of the North American continent, which extended from the 

 old Appalachian region across the present Indian Territory and 

 New Mexico to the Mexican province of Sonora. 2 



In the middle of the Cretaceous period (at the end of the 

 Comanche series, Gault) a large part of Mexico became land, 

 forming a southern continuation of the western part of North 

 America, which was separated in the Upper Cretaceous from the 

 eastern, and which therefore extended from British Columbia 3 to 

 the Isthmus of Tehuantepec. This strip of land formed during this 

 period a very important barrier, separating the marine faunas of 

 the Pacific and Atlantic Oceans. While both faunas were more or 

 less connected during the Lower Cretaceous across Mexico, they 

 became separated later and never again communicated in this 

 region. 



The Isthmus of Tehuantepec consists, according to Spencer, 4 of 

 the identical Lower Cretaceous deposits found in Mexico, and, 

 further, according to Sapper, 6 Cretaceous rocks are found in the 



1 This barrier was probably emphasized by the development of desert condi- 

 tions in and at the foot of this mountain range. Compare Scott, W. B., An In- 

 troduction to Geology, 1897, p. 500: " Probably the upheavals at the end of the 

 Bridge r and at the end of the Eocene had made the climate much drier by cut- 

 ting off the moisture-laden winds." 



2 In 1898 (pp. 243 and 259) Hill qualifies his views, and says that it is doubt- 

 ful whether the whole country (Mexico) was entirely submerged at anyone time 

 during this period. He thinks it was a mere shifting of the barrier between the 

 Atlantic and Pacific. Compare, also, Stanton, T. W., in Journ. of Geology, Vol. 

 3, 1895, p. 861. 



3 And these parts must have been connected, as we have seen above, with 

 Northeastern Asia. 



4 Bull. Geolog. Soe. America, Vol. 9, 1897. 



5 Boll. Instit. Geol., Mexico, Vol. 3, 1896. 



