256 CONFERENCE ON PALEOZOIC PALEOGEOGRAPHY 



lift, which form a part of the western shore. The significance of such 

 facts are apparent when we note the relatively short distance separating 

 Missouri and southern Illinois from the Kashville and Cincinnati domes, 

 and the much greater distance between western Tennessee and Ontario, 

 along which the deposits of this age are filled with Bryozoa and corals. 



From the facts just stated it is inferred that a marine current entered 

 from the Gulf and, hugging the eastern shore, carried the free-swimming, 

 larvae of these sessile types as far as it was competent. In the case of the 

 Stones Eiver this current seems to have spent its force before reaching 

 Virginia. In the other two cases, the Lowville and Onondaga, the cur- 

 rent continued on through New York and Ontario. 



More or less similar invasions of southeastern American continental 

 basins occurred in succeeding Paleozoic ages. Notable among them is 

 the early Trenton Wilmore limestone fauna, which contains a number of 

 Bryozoa ranging from Kentucky to Canada; second, the late Eden and 

 Maysville faunas, which contain numerous forms in zones which can be 

 followed up the Appalachian Valley from Tennessee to New York and 

 southern Ontario, where they are represented in the Lorraine formation ; 

 and, third, the late Clinton Eochester shale, which has a similar distribu- 

 tion. A somewhat different distribution of bryozoan faunas invading 

 from the south is shown by the earliest Clinton, which extends, like the 

 Lorraine preceding it, from Tennessee to southern Canada in a north- 

 easterly direction, but also extends to Oklahoma in a western direction. 

 The Helderbergian fauna, which includes many Bryozoa, is a good ex- 

 ample of a southern Atlantic fauna invading separate continental basins, 

 in the one case through the Mississippi embayment, and in the other 

 from the middle Atlantic to the Appalachian troughs by way of an in- 

 ferred opening at Chesapeake Bay. 



Finally, the late Tennessean Chester Bryozoa, which passed northward 

 in and across the Atlantic to England, where they are represented in the 

 Mountain limestone, and which, on the American side, spread through 

 the continental basins of southeastern United States from northern 

 Arkansas to Maryland. 



The invasions from the north — that is, from the Arctic basin — are 

 similarly and no less readily discriminated than are the southern inva- 

 sions. Like them, the faunas range as far south as their respective beds 

 extend, terminating their geographic distribution when these beds wedge 

 out by overlap. There are two important Ordovician bryozoan faunas 

 which evidently originated in the Arctic and spread southward into the 

 basins of North America, The first of these is best known from the late 



