444 C. SCHUCHERT PALEOGEOGRAPHY OF XORTH AMERICA 



faunas become very widespread. As examples may be cited the Antillian 

 shallow-water fauna living within 60 fathoms, which has spread to Per- 

 nambuco, Brazil. Florida, and Bermuda, and a few species even to cape 

 Hatteras. The same is true in the Pacific ocean from north Peru to 

 southern California, and these are distances representing 3,000 miles or 

 more. In this wide distribution undoubtedly the currents have greatly 

 facilitated migration and the blending of faunal elements, yet a far 

 greater factor is an equable oceanic temperature throughout the year. 

 Similar disjDersion may be noted in the case of single species : for instance, 

 that of Purpura lapillus. 



■'This group is abundant in the North Atlantic, and has made its way through 

 i:he Boreal region into the Pacific, being modified into several geographic races. 

 On the western coast of North America, where there are no sudden changes in 

 the temperature of the sea water, this group has made its war as far south as 

 "Margarita Bay. in latitude 24° N.. mean temperature 23° C. On the Asiatic 

 side it has made its way through Bering Sea down the shores of Kamschatka 

 with the cold water, but has been stopped by the sudden change of temperature 

 at Hakodadi, latitude 41° X.. Japan, mean temperature 11° C. where the warm 

 Japan current meets the cold current from Bering Sea. That this is not an 

 accident of distribution is shown by the fact that the group of Purpura lapillus 

 has, in the Atlantic, a similar distribution, and for the same reasons. On the 

 African side it reaches latitude 32° N. 5 mean temperature 19° C, and on the 

 American side it is barred back by the sudden change of temperature at lat. 

 42° X.. mean temperature 11° C. There can be no doubt that the temperature, 

 or rather evenness of change of temperature, controls the distribution of Pur- 

 pura lapillus now'* (J. P. Smith, von Koenen. Festschrift. 1907: 415). 



During periods of greatest inundation in warm climates, as those of the 

 Mohawkian and the Xiagaran of the Mississippian province and of the 

 Middle Devonic of the Euro-Asiatic province, there are in America almost 

 universal faunas. This is also in conformity with the postulate that dur- 

 ing times of extensive inundation the land barriers are least effective. 

 That the physical conditions of the Paleozoic can not be altogether deter- 

 mined by the character of the present Atlantic may be proved by the tabu- 

 lations of shallow-water life presented by Doctor Dall. He states that in 

 the existing "cool temperate zone,*' where the minimum winter tempera- 

 ture of the water is not below 40 degrees Fahrenheit, in both the Xorth 

 Atlantic and the Pacific ocean, and at any station where good collections 

 have been made, there live on the average 40? shell-bearing molluscan 

 species. In the "warm temperate zone/ 7 having a temperature of between 

 60 degrees and TO degrees Fahrenheit, the average is 483 species, while in 

 the tropical zone the average is 629 species. It is also stated that these 

 figures compare favorably with the number of species representing Ter- 

 tiarv faunas, but that with the latter the tendencv is toward even larger 



