THE MISSISSIPPIAN PERIOD 613 



of the interior. It flourished while the St. Louis and Kaskaskia 

 formations were being deposited. 



The Genevieve (St. Louis-Kaskaskia) fauna. As already 

 stated, 'the St. Louis formation marks the stage of maximum sea- 

 extension in the interior of North America, and the Kaskaskia 

 deposits represent a narrowing and shallowing sea. The Genevieve 

 fauna, representing the two stages, may be regarded as including 

 the culmination of the cosmopolitan evolution of the marine life 

 of the Mississippian period on the North American continent, and 



Fig. 432. Genevieve Echinoderms: a, Agassizocrinus dactyloformus Shum., 

 a crinoid which lost its stem and became a free swimming creature, 

 at least in its adult condition; b, Acrocrinus amorpha W. and Sp., a 

 specialized camerate crinoid with a large number of supplementary plates 

 introduced between the basal and radials; c, Pentremites robustus Lyon, 

 a blastoid. 



the initiation of its decline. The commingling of the Great Basin 

 and the Osage faunas was the most distinctive feature of the Gen- 

 evieve fauna. It introduced into the main Mississippian sea what 

 seemed to be a retrograde change, for species of a Devonian aspect 

 that still lived in the isolated Great Basin province and elsewhere, 

 migrated eastward, and their relics are found with species whose 

 evolution had reached an advanced Mississippian phase. 



Crinoids were less plentiful than in the Osage fauna, and notably 

 changed (Fig. 432). Of one group which had upwards of 300 

 species in the Osage fauna, less than 25 species are known in the 

 Genevieve, and among the 25, no Osage species persisted. Other 

 groups of crinoids, however, did not show so remarkable a decline 



