538 C. SCHUCHERT PALEOGEOGRAPHY OF NORTH AMERICA 



nensis. This is followed by the Ohio Clinton horizon of wide extent, 

 from Oklahoma to Ohio, marked by Triplecia ortoni, Stricfclandinia 

 triplesiana, Atrypa margindlis, Orthis flabellites, Plectambonites trans- 

 versalis, Rhinopora verrucosa, Phamopora magna, Pachydictya bifurcata, 

 Favosites favosus, Halysites catenulatus, Stromatopora, etc. In the Anti- 

 costi section it is seen that this so-called Clinton, or the Triplecia ortoni 

 zone, is older than the true Clinton of the Appalachian region. The 

 older Clinton, therefore, is here mapped as the Ohio Clinton. Ulrich has 

 discovered that in northeastern Alabama this same fauna underlies the 

 Appalachian Clinton with the Anoplotlieca hemispherica fauna. 



These are the early Sihiric faunas of the Gulf region, the first oscilla- 

 tions that finally terminated in the Louisville transgression. In the 

 earliest stages of this transgression not more than 6 per cent of the North 

 American continent was covered by the continental seas, and about 12 per 

 cent of the United States. At the height of the invasion about 36 per 

 cent of both areas was inundated. The waters finally receded somewhat 

 beyond the stage represented at the opening of the Siluric, for Manlius 

 time shows the figures to be 5 and 10 per cent respectively for the two 

 areas above mentioned. 



The various formations that record this transgression first appear in 

 the Mississippian sea, and later in the Hudson- Arctic sea. They are 

 arranged in the Siluric table. 



The Guelph faunas are distinguished from those of the Louisville and 

 equivalent horizons by an almost complete absence of cystids and crinoids. 

 The corals are scarce and are of the genera Favosites (3 species), Syringo- 

 pora (1), Halysites (2), HeJioHtes (1), and Stromatoporoidea (6). Of 

 brachiopods, practically no other forms are found than Trimerella (5 spe- 

 cies), EMnobolus (2), Monomorella (6), Pentamerus oblongus, Con- 

 chidium occidentale, and Clorinda ventricosa. The guiding pelecypod is 

 MegaJomus canadensis. The Guelph is especially distinguished by gas- 

 tropods, of which there are more than 50 species. Many of these are 

 large and nearly all are thick shelled. Of nautiloids, there are at least 20 

 species belonging to genera known in the Eacine. Large Leperditia are 

 also present. The trilobites are not conspicuous, and Eurypterus is 

 rare. 157 



Cayugan emergence. — The Xiagaran transgression attained its maxi- 

 mum spread toward the close of Louisville time. Early in the Guelph 

 the sea began to retreat, first in the Appalachian trough and next in the 

 Indiana basin, where the Decatur is thought to be of this time. A little 



157 Whiteaves : Paleozoic Fossils, vol. 4. 1906, and Clarke, Memoir no. 5, New York 

 State Museum, 1903. 



