528 GEOLOGY. 



This distinctive phase of the Great Basin fauna was contemporaneous 

 with the Kinderhook and. Osage faunas of the interior sea, and at the 

 close of the Osage epoch it united with the latter to form the Genevieve 

 (St. Louis-Kaskaskian) fauna. 



Previous to this union, one of the most salient distinctions between 

 the Great Basin fauna and the Osage fauna was the rarity of crinoids 

 in the former. Although the Osage sea stretched westward to the 

 foot of the ancestral Rocky Mountains, and was prolific in crinoids, 

 they do not seem to have invaded the Great Basin sea, one of the 

 facts which point to the distinctness of the two provinces. Brachio- 

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

 The large spirifers of the type of Spirifer striatus (compare Fig. 236, m), 

 so characteristic of the Osage fauna and of the "mountain limestone" 

 of Europe, as well as other typical forms, were absent. On the other 

 hand, there had arisen, under the genus Productus, probably by par- 

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

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

 tive way, in the Mississippian progress. Were it not for such forms 

 of Mississippian aspect, and the evidence that developed when the 

 two faunas commingled, the Great Basin fauna might be thought 

 to be Devonian, for there remained many species of marked Devonian 

 aspect. Among these was Leiprhynchus quadricostatus, known at the 

 east only in Hamilton beds, but in the Great Basin found to commingle 

 in the upper horizons with Mississippian types. Productella, a Devo- 

 nian type, was still living in the Great Basin, although it had disap- 

 peared from the interior after the Kinderhook epoch (Fig. 235, n). 

 Numerous other species of Devonian aspect were associated with forms 

 of Mississippian aspect. 



Judging from present knowledge, the pelecypods were even more 

 abundant than the brachiopods, another point of contrast with the 

 Osage fauna. The pelecypod species seem to have been peculiar to 

 the Great Basin sea, and doubtless originated there, and later became 

 extinct without migration. There were also many gastropods, among 

 which were air-breathers (Physa prisca and Zaptychius carbonaria), 

 the oldest aquatic pulmonates known. Terrestrial pulmonates, theo- 

 retically evolved from aquatic pulmonates, have, however, been de- 

 scribed from Devonian strata. Cephalopods were not abundant, and 

 no goniatites have thus far been found among them. Trilobites were 



