STRATIGRAPHIC CLASSIFICATION PALEONTOLOGIC CRITERIA 485 



Ordovician and Silurian; also in later Paleozoic ages. The assumed land 

 connection which made these migrations possible served to separate the 

 middle Atlantic fauna from the Arctic fauna in the North Atlantic basin. 



The migrations of the pelagic graptolite faunas of the Canadian (Tetra- 

 graptus) and Ordovician (Nemagraptus) on the contrary suggest either a 

 south extension of the Baltic trough, which in that case may have con- 

 nected the Arctic and middle Atlantic basins, or a partial submergence of 

 the intercontinental land connection. In either case restriction, if not 

 complete prohibition, of direct littoral migration must have resulted, 

 while current transportation of the floating organisms would have been 

 favored. The Helderbergian invasion of Europe probably followed a 

 similar path except that instead of passing into the Baltic trough, which 

 must have been emerged at this time, it turned southward into newly 

 submerged areas now occupied by resulting lower Devonian deposits. 

 But it is not impossible that this invasion was accomplished by migration 

 from the south Atlantic along the east shore, and thence through the 

 Mediterranean into west central Europe. 



Eegarding later Paleozoic migrations across the Atlantic, the Neo- 

 devonian sea of New York is thought to have been in direct though per- 

 haps not very favorable communication with the German seas of this time 

 by way of the supposed north shore of the middle Atlantic, the Few 

 England coast, and finally an opening between Appalachia and Taconia 

 (New Jersey Strait) connecting the Atlantic with the Appalachian troughs 

 and the more inland New York sea. Appalachia and the Antilles at 

 this time probably were united by land connection. The Atlantic Waver- 

 lyan, Tennessean, and Pennsylvanian faunas at times followed the same 

 path between Great Britian and the Appalachian Valley, but more com- 

 monly, instead of crossing Appalachia, they passed through an opening 

 between the Antilles and the Floridian extremity of Appalachia into the 

 Gulf of Mexico and thence up the Mississippi embayment. 



Silurian routes of migration. — Although the pre-Devonian European 

 faunas invaded North America chiefly by following the shores of invading 

 Arctic seas, a number followed the Atlantic route and thence passed 

 through one or more of the openings between the marginal lands. These 

 openings were closed during most of the Niagaran and at other times, 

 but one or another was used by many Ordovician and certain Clinton and 

 Cayugan faunas that are represented in England and western Europe, 

 but not in northern Europe. Somewhat different communication with 

 Europe occurred when these eastern openings were closed. Thus some 

 of the Baltic Silurian genera of crinoids and trilobites seem to have 

 passed freely to the southwestward into the middle Atlantic basin, thence 



