410 GEOLOGY. 



with Europe, Asia, Australia, and New Zealand may be assumed as 

 established. Since thirty or more species x are known to be common 

 to North America and Europe (the interior of North America and 

 Sweden and Great Britain), and since these embrace a wide range of 

 genera of very different habits, there is a strong presumption of rather 

 intimate relations. The lines of intercommunication therefore become 

 matters of interest biologically and geographically. 



The migratory route. — The most marked community of species is 

 observed, not between the Atlantic border fauna of America and that 

 of the opposite coast of Europe, but between the interior American 

 fauna and that of Sweden and Great Britain. Singularly enough, 

 it is the interior fauna that is pronouncedly intercontinental and cos- 

 mopolitan in character. It has already been stated that the interior 

 sea seems to have had no free communication with the eastern and 

 southern coast regions, and that an extension to the far west is doubt- 

 ful, while to the northward a chain of deposits, now separated by ero- 

 sion but probably once continuous, points to a broad thoroughfare 

 in that direction (see map Fig. 174 and Fig. 190). This reaches to 

 North Greenland, from which, by way of Spitzbergen, Nova Zembla, 

 and North Russia to Gothland in the Baltic, the steps are not excessively 

 great. The supposed connecting fauna has not, however, yet been 

 identified in the imperfectly known Paleozoic beds of Spitzbergen> 

 Nova Zembla, and North Russia. There is a possible alternative by 

 way of the Iceland-Scotland submerged plateau, and still another 

 alternative by way of the northwestern or Siberian route. 



The paleontologic evidence favors a northern route, for the fauna 

 of the Arctic islands contains many species common to the American 

 interior province and the Swedish-English province. It is significant 

 that the Mid-Silurian fauna of Sweden more closely resembles that 

 of the American interior than it does that of New York. Relative 

 to the assigned northern thoroughfare, the New York area was a side- 

 gulf or embayment less readily reached by immigrants from the north 

 than were the more open seas to the west. The very singular com- 

 munity of aspect recently shown to exist between the faunas of Goth- 

 land, Sweden, and Dudley, England, and the Iowa-Illinois- Wisconsin 

 region give special emphasis to this. 2 Crotalocrinus, one of the most 



1 Phillips, Man. Geol., Pt. II, 1885, p. 122. 



2 Weller, Jour of Geol, Vol. IV, 1896, pp. 166-173, and Vol VI, 1898, pp. 692-703. 



