THE SILURIAN PERIOD. 411 



singular and highly specialized genera of crinoids, together with the 

 genus Coryiribocrinus, are common to the Gothland and the Chicago 

 faunas. Petalocrinus, another highly specialized genus, is common 

 to Gothland, Iowa, and Indiana. The peculiar little twisted brachio- 

 pod Streptis, of Europe, also occurs at Batesville, Ark., as do the 

 peculiar trilobites Deiphon and Stauroce halus, which occur also at Chi- 

 cago. The notable case of the square-topped operculated coral has 

 already been mentioned. The presence of these several very peculiar 

 and specialized forms at these localities on the two continents, taken 

 in connection with the many less peculiar forms, clearly implies free 

 intermigration and points with probability to the northern route. 



The faunas that occupied the Appalachian trough, the St. Lawrence 

 embayment, and the Atlantic coast appear to have had only indirect 

 and limited connection with that of the interior during the earlier and 

 Mid-Silurian epochs. 1 In the present state of knowledge, these bear 

 the aspect of provincialism, as they have only a few species com- 

 mon to the great cosmopolitan fauna of north central America and 

 northern Europe. 



The Closing Restrictional Stage. 



Following the luxuriant life of the Mid-Silurian epoch, there came, 

 in North America at least, a very notable decline due to the with- 

 drawal of the epicontinental waters from the larger part of the interior 

 and to the conversion of the remainder into an excessively salt sea. 

 In the deposits (Salina) of this sea almost no fossils are found The 

 tract of excessively briny water appears to have formed a veritable 

 " dead sea," though it was perhaps not absolutely devoid of life. This 

 almost - lifeless sea was a peculiar and rather local development, and 

 is to be regarded as a striking incident only in the more general restric- 

 tional movement. In addition to this special feature, the life evolu- 

 tion appears to have been affected by the partial replacement of the 

 marine fauna by a fresh or brackish- water fauna, another unusual fea- 

 ture, at least one not before clearly recorded. It cannot be said, how- 

 ever, that this was certainly true, but it appears to us the better of the 

 two interpretations that will presently be set forth. If this be correct, 

 the rich marine fauna which prevailed so widely in the Mid-Silurian 

 times suffered restriction in the closing Silurian stages not only to the 



1 Ulrich & Schuchert Rep. N. Y. St. Pal., 1901, pp 647-651. Weller, Geol Surv. 

 N. J Pal, Vol III, 1901, pp. 72-76. 



