THE MISSISSIPPIAN PERIOD 



373 



FIG. 389. One of 

 the last represen- 

 tatives of the trilo- 

 bites (Phillipsia) 

 found in Mississip- 

 pian rocks. 



noids or both. They were preeminently animals of the clear 



seas ; but the conditions of depth, temperature, and food 



supply which are essential for one are not 



quite those which are required for the other. 



In some parts of the Mississippian sea of 



the United States, corals seem not to have 



been favored, although they were common 



elsewhere. Where the corals were few, 



crinoids were locally so abundant that some 



strata are composed mainly of their stems 



and scattered plates. At no time in their 



history were the crinoids 



more diversified or more 



highly ornamented. 



Like the trilobites of the 



Silurian, some of them 



assumed eccentric and 



seemingly useless changes 



of form, with spines, ridges, and knobs upon 



the plates. Similarly, the crinoids were at 



this time on the verge of a rapid decadence ; 



by the close of the Mississippian period the 

 lG ;.u;T c m ~ majority of them had become extinct, 



pleteblastoid. One J . J 



of the stemmed leaving a decreas- 



echinoderms, espe- mg J me Q f d escen d- 

 cially common in . 



the Mississippian ants which are but 



limestones. poorly represented 



in our modern seas. The cause of 

 their decline is yet a mystery. 



Development among the fishes. 

 In the eastern part of the interior sea, 

 fishes were numerous, and, as we may 

 well believe, the most formidable 

 predatory animals of the time. Sig- 

 nificant changes had taken place FlG - ? 91 .- ~ A 



. . gomatite with moderately 



among them since the Devonian. The folded sutures. 



