434 



MISSISSIPPIAN PERIOD 



was a favorable field for the evolution of the varied assemblage of 

 forms that had come together in preceding epochs under less favor- 

 able conditions. There is evidence also of rather free migratory 

 communication with the Eurasian continent, since many species 

 were common to America and Europe. 



No single group so well characterizes the Osage fauna and ex- 

 presses its dependence on physical conditions as the crinoids, whose 

 abundance and diversity were climacteric (Fig. 378). Their rapid 



Fig. 378. OSAGE ECHINODERMS: a-d, crinoids; a, Barycrinus hoveyi Hall; b, 

 Eretmocrimis remibrachiatus (Hall), having spatulate arms; c, Actinocrinus lobatus 

 Hall, shows highly ornamented plates; d, Forbesiocrinus wortheni Hall, a flexible 

 crinoid; e, a blastoid ; Oligoporus mutatiis Keyes. 



decline after this epoch is one of the most remarkable incidents in 

 the life history of the invertebrates. In the day of their glory, the 

 crinoids were most prolific, as indicated by the fact that a single 

 genus (Batocrinus) , had more than a hundred species. Their orna- 

 mentation was notable, and as in the case of the trilobites, preceded 

 their decline. The repetition of this phenomenon at different times 

 and in different groups of organisms is worthy of notice, though its 

 meaning is not altogether clear. Crinoids made large contributions 



