STRATlGllAPHiC CLASSIFICATION PALEONTOLOGIC CRITERIA 489 



different from those found in northeastern Canada and Minnesota. 

 Faunas of Canadian age being entirely unknown in northeastern North 

 America, the small early Canadian faunas procured by Mr. E. M. Kindle 

 in Seward peninsula, Alaska, throw no direct light on this question. It 

 should be said, however, that so far as they go these Seward peninsula 

 species are exceedingly suggestive of the Ceratopyge fauna of Sweden. 



The Pacific faunas. — The third center of origin and distribution of 

 faunas lay in the great Pacific basin. Considering the whole of geological 

 time and that its waters sometimes spread widely over Asia and Europe 

 on the one side and North America on the other, it is perhaps the most 

 important of the three faunal realms here recognized. In part, it corre- 

 sponds to Frech^s Pacific- American basin, ^" but he includes in this center 

 of distribution the Gulf of Mexico basin, whose fauna I believe to have 

 been as a rule quite distinct from those of the Pacific basin proper. It 

 is true that at certain times of great submergence, as in the Canadian, 

 the Black River, the Richmond, and the late Devonian, the Pacific faunas 

 invaded areas in the Mississippi Valley more commonly occupied by Gulf 

 of Mexico waters, and that, as in the middle Waverlyan, the southern 

 waters occasionally spread farther west than usual. But it is no less true 

 that the Arctic faunas also invaded these areas either by direct southward 

 extension across the North American continent or by means of a Baltic 

 communication with the middle Atlantic, and through this with the Ap- 

 palachian Valley and the Mississippi embayment. 



In fact, the transgressions of the several oceanic waters varied greatly 

 from time to time, certain of the interior continental basins being repeat- 

 edly captured and released by first one and then another of the main seas. 

 Because of the bearing of paleogeography on correlation and stratigraphic 

 taxonomy the determination and delineation of these varying successive 

 invasions affords the most important as well as the most interesting of 

 the duties of the modern stratigraphical paleontologist. 



The fossil record of the evolution of the Pacific fauna, while very full 

 in certain parts, is in other parts much less complete than that of the 

 Atlantic fauna. Thus the evolution of the Cambrian faunas of the 

 Pacific is more continuously and probably more fully recorded in the enor- 

 mous deposits of this age in the Cordilleran trough of western North 

 America and in southeastern Asia than is that of either the Atlantic or 

 the Arctic Cambrian faunas. 



The ollenellid trilobites probably originated in the Pacific, but as cer- 

 tain species of this genus became very abundant in tlie Atlantic fauna we 



«> Lethsea Paleozoica, vol. 2, 1897, p. 88. 



XXXIII— Bull. Geol. Soc. Am., Vol, 22, 1910. 



