436 



MISSISSIPPIAN PERIOD 



Kinderhook and Osage types, and took on slowly a Mississippian 

 aspect, while retaining many Devonian characteristics. Its most 

 prominent members were the pelecypods, as might have been antici- 

 pated from the silty conditions. 



The Great Basin fauna of the first half of the period records a 

 gradual evolution of the Devonian fauna of the same region, with 

 perhaps the addition of a few immigrants from the west. After 

 the Osage epoch, the Basin fauna united with the Osage fauna of 

 the interior, and this union had an important effect on the later 

 Mississippian faunas of the interior. 



Previous to the union, the salient features of the Great Basin 

 fauna were the (i) rarity of crinoids; (2) among brachiopods the 

 absence of spirifers, so characteristic of the Osage fauna, and the 

 presence of the genus Productus, closely allied to species of the Osage 

 fauna and probably developed by parallel evolution; (3) the pre- 

 ponderance of pelecypods over brachiopods; (4) the abundance of 

 gastropods, among which were air-breathers, the oldest aquatic 

 pulmonates known; and (5) plentiful cor- 

 als, the horn-shaped type predominating. 

 Cephalopods and trilobites were few, and 

 no 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 pro- 

 vince, for fish abounded in the eastern sea. 

 The barrier which separated the Great 

 Basin and the Kinderhook-Osage seas 

 appears to have been an elongated insu- 

 lar tract lying between the Rocky Moun- 

 tains and the Great Basin. The yielding 

 of this barrier about the close of the Osage 

 epoch, by erosion or submergence, permit- 

 ted the singular semi-Devonian, semi- 

 Mississippian fauna of the west to invade 

 the greater eastern sea. The late Missis- 

 sippian (St. Louis) faunas of the interior 

 include (i) the culmination of the cosmo- 

 politan evolution of the marine life of the 

 Mississippian period on the North Ameri- 

 can continent, and (2) the initiation of its 



Fig. 380. UPPER MIS- 

 SISSIPPIAN ECHINODERMS: 

 a, Agassizocrinns dactyli- 

 formis Shum., a crinoid 

 which lost its stem and 

 became a free swimming 

 creature, at least in its 

 adult condition; b, Acro- 

 crinus amphora W. and Sp., 

 a specialized camerate cri- 

 noid with a large number 

 of supplementary plates 

 introduced between the 

 basal and radials; c, Pen- 

 tremites robustus Lyon, a 

 blastoid. 



