612 GEOLOGY 



points to the distinctness of the two provinces. Brachiopods were 

 abundant in both provinces, but the species were different. The 

 large spirifers so characteristic of the Osage fauna and of the 

 " Mountain Limestone " of Europe were absent. On the other hand, 

 there had arisen, under the genus Productus, probably by parallel 

 evolution, species closely allied to some found in the Osage fauna, 

 implying that the Great Basin fauna was taking part, in a conserv- 

 ative way, in evolutionary progress. Were it not for such forms 

 of Mississippian aspect, and the evidence that developed when the 

 two faunas commingled, the Great Basin fauna of this time might 

 be thought to be Devonian, for many species of marked Devonian 

 aspect remained. 



Another point of contrast with the Osage fauna was the abun- 

 dance of the pelecypods as compared with the brachiopods. The 

 pelecypod species seem to have been peculiar to the Great Basin 

 sea, and doubtless originated there, and finally became extinct 

 without migration. There were also many gastropods, among which 

 were air-breathers, the oldest aquatic pulmonates known, though 

 terrestrial pulmonates have been described from Devonian strata. 

 Cephalopods were not abundant. Trilobites were about as rare as 

 in the eastern fauna, and of the same genera. Corals were present 

 in some abundance, the horn-shaped type predominating. Neither 

 bryozoans nor fishes have been reported. Unless this is 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 en- 

 forced the distinctness of the Great Basin and the Kinderhook- 

 Osage seas appears to have been an elongated insular tract lyin^ 

 between the Rocky Mountains and the Great Basin. The yielding 

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

 submergence, permitted this singular semi-Devonian, semi-Mi 

 sippian fauna of the west to invade the greater eastern sea. The 

 commingling of the two faunas gave rise to the Genevieve l fauna 



1 The Genevieve fauna is not restricted to the St. Genevieve limestone. 



