THE MISSISSIPPIAN PERIOD. 529 



about as rare as in the eastern fauna, and of the same genera. Corals 

 were present in some abundance, the horn-shaped type predominating. 

 No bryozoans have been reported, nor have any fishes been described. 

 Unless this be due to the imperfection of the record, or of present 

 investigation, it adds much to the evidence of the distinctness of the 

 province, for fish abounded in the eastern sea, and they are free-moving 

 forms of migratory habits. 



As remarked in the physical discussion, the barrier which enforced 

 the distinctness of the Great Basin and the Kinderhook-Osage seas 

 appears to have been an elongated insular tract lying about where 

 the Rocky Mountains (not then elevated) now stand. The yielding of 

 this barrier about the close of the Osage epoch, by erosion or sub- 

 mergence, permitted this singular semi-Devonian, semi-Mississippian 

 fauna to invade the greater eastern sea, and, commingling and com- 

 bating with the Osage fauna, to initiate a new fauna, the Genevieve, 

 the closing fauna of the Mississippian period hx the interior. It 

 flourished while the St. Louis and Kaskaskia formations were being 

 deposited. 



The Genevieve (St. Louis-Kaskaskia) Fauna. 



As remarked in the physical discussion, the St. Louis formation 

 marks the stage of maximum sea-extension in the interior of North 

 America, and the Kaskaskia deposits the inauguration of a restrictive 

 movement. The Genevieve fauna, representing the two stages, may 

 be regarded as the culmination of the cosmopolitan evolution of the 

 Mississippian life on the North American continent, and the initia- 

 tion of its decline. The commingling of the Great Basin and the 

 Osage faunas was the most declared feature. It introduced into the 

 main Mississippian sea what seemed to be a retrograde feature, although 

 it was a part of the progressive movement toward a more cosmopolitan 

 fauna. Species of a Devonian aspect that had lived along conserva- 

 tively in the isolated Great Basin province, migrated eastward, and 

 were found among species whose evolution had reached an advanced 

 Mississippian phase. The •faunal average seemed to be set backwards. 

 Among these were several productellas, and Leiorhynchus qnadricostatus, 

 previously mentioned as lingering Devonian forms in the Great Basin 

 fauna. That these really came from the west seems to be confirmed 



