254 CONFERENCE ON PALEOZOIC PALEOGEOGRAPHY 



geographic pattern of some particular time period continental basins 

 connecting in the case of North America with the Arctic on the north, 

 the Atlantic and Pacific on the east and west, and still others that were 

 occupied by waters invading from the Gulf of Mexico. 



Possibly these different oceanic waters contributed to continental basins 

 at the same time, but as a rule this would seem to be highly improbable. 

 Instead it is thought that when seas were entering the southern border 

 and filling certain continental basins the northern waters were excluded. 

 Whether solely by tilting of the surface of the continent, or whether 

 abundant heaping of oceanic waters toward the equator and then back to 

 the poles contributed in any marked degree, is not readily determinable 

 and is, after all, beyond the scope of the present paper. Under this con- 

 ception it follows that the same basin often contains superposed deposits 

 and faunas originating in the Arctic, the Atlantic, and the Gulf of 

 Mexico, and occasionally, as in Oklahoma, in waters from all four sides. 



The Paleozoic basins now included within the Mississippi Valley 

 usually alternated between the Gulf of Mexico and Arctic waters, but 

 these, so far as known, were never present at the same time and there- 

 fore never mingled. In the Appalachian region, however, where Gulf 

 and, more rarely, Arctic waters alternate with Atlantic invasions, con- 

 fluence of the first and last and consequent mingling of the faunas is 

 occasionally suggested. 



However, even in these instances the community of species in other- 

 wise typical north Atlantic and Gulf faunas may be more plausibly ex- 

 plained on the assumption that these species ranged in the south Atlantic 

 as well as the north Atlantic, hence invaded from both directions. There- 

 fore, without going into a detailed statement of the facts on which the 

 opinion is based, it is concluded that the several oceanic waters and 

 faunas seldom if ever intermingled within continental basins. 



As said, the Bryozoa began to constitute a very considerable proportion 

 of the marine faunas of the continental seas in the middle Stones Eiver, 

 a group of Ordovician rocks that is well developed in central Tennessee 

 and attains much greater thickness in the Appalachian Valley. The 

 Bryozoa are especially abundant in the Pierce division of the group, a 

 bed with a maximum thickness of 27 feet in central Tennessee, but at- 

 taining much greater dimensions in the southern Appalachians. The 

 Bryozoa characterizing the Pierce consist chiefly of bifoliate Cryptosto- 

 mata belonging to the Ptilodictyonidse and Ehinidictyonidae. In central 

 Tennessee, also in the southern Appalachians in Alabama and eastern 

 Tennessee, these forms are exceedingly abundant, but in following the 

 beds northward in the Appalachian Valley they rapidly diminish in num- 



