38 IXTRODUCTION 



Straits. Another most marked oceanic opening was the Gulf of Mexico 

 embayment of the Mississippi Valley, but until late Middle Devonian 

 time none of its faunas reached Maryland. Then and subsequently the 

 influence of this embayment was slight as far as the composition of the 

 Appalachian faunas was concerned. The Helderberg life of Oklahoma, 

 Alabama., Tennessee, and Illinois, however, closely resembled the typical 

 development in the Appalachian Sea but was very different from that of 

 the St. Lawrence Sea. In northern Europe the Helderberg faunas are 

 unknown, but in southern Europe, in Bohemia, they are present in the 

 Konieprusian limestone. While but few species are common to the two 

 sides of the Atlantic, it is certain that all belong to the Atlantic realm and 

 that the southern migration took place along the northern shore of 

 Gondwana. The same general fauna, but much changed, doubtless by 

 latitude and somewhat cooler waters, occurs in the St. Lawrence Sea. 

 These forms may have distributed themselves around the shores of the 

 North Atlantic, this surely being the path of migration during the 

 Oriskany. Clarke has termed these migrations " the Coblenzian in- 

 vasion," its faunas being well known in northern Europe. 



The Helderierg Map (Plates V and VI). — The Helderberg series 

 begins with the Keyser limestone and continues through the New Scot- 

 land into the Becraft. Underlying these in the Appalachian trough of 

 Maryland is a thick series of limestones — McKenzie, Wills Creek, and 

 Tonoloway formations— of late Silurian time. Both series are character- 

 istic of the southern Appalachian trough, but are unknown over the 

 greater portion of the North American Continent. The Helderberg 

 faunas, as such, do not make their appearance before the New Scotland, 

 yet even in the upper part of the Tonoloway, forms occur, which an- 

 cestrally are clearly allied to those of the formation first mentioned. As 

 the New Scotland and Becraft faunas, however, are almost identical with 

 those of eastern New York, the typical area for the Helderberg, and are 

 somewhat unlike those of the southern states, the areas evidently had no 

 direct intercommunication, although both belong to one faunal realm — 

 the South Atlantic in connection with the Mediterranean. The Maryland 

 Helderberg faunas came into this region by way of the New Jersey 



