THE MISSISSIPPIAN PERIOD, 523 



is observable in all of the more prominent families, and seems to be 

 a clear case of parallel evolution. The ornamentation was effected by 

 the development of surface ridgings, nodes, spines, and other out- 

 growths from the plates. 1 It is even possible to recognize a distinct 

 advance in the ornamentation during the progress of the epoch. In 

 the earlier (Burlington) stage the crinoids were usually smaller and 

 more delicately ornamented than in the later (Keokuk) stage, when 

 they attained larger sizes, assumed broader features, and took on 

 more extravagant ornamentation. The meaning of all this is not 

 clear, but the repetition of this singular phenomenon at different times, 

 and in quite different groups of organisms, is worthy of notice and 

 thought. 



The crinoids made large contributions to the limestones, the char- 

 acteristic deposit of the epoch, the "encrinital limestone " taking its 

 name from the numerous plates and stems which make up much of 

 its substance. 



The distant kin of the crinoids were not equally affected by the 

 conditions of the epoch, although the blastoids were represented by 

 a greater variety of forms than at any other period in their history. 

 The starfishes, though present, did not appreciably depart from the 

 slow development that had been their habit for ages, and much the 

 same is to be said of the brittle stars that were even less common. 

 The echinoids showed a slightly greater movement, and left a record 

 of about a score of species. 



The scantiness of the corals. — It is a matter of surprise that the 

 corals had so small a place in this fauna, in view of the general physical 

 conditions which were apparently favorable, though there may have 

 been depths or temperatures uncongenial to them (Fig. 236, u). 

 Crinoids are less narrowly circumscribed in these respects than corals. 

 It is more likely, however, that the explanation lies in organic con- 

 ditions, perhaps in the state of the corals themselves, perhaps in unre- 

 corded enemies, parasitic or otherwise, or perhaps in the pre-occupation 

 and rivalry of the crinoids. 



Other features. — The brachiopods, as usual, held a leading place 



1 Admirable illustrations may be found in the Monograph of Wachsmuth and 

 Springer on "North American Crinoidea Camerata," Mem. Mus. Comp. Zool., Vols. 

 20-21, and also in the state reports of Iowa, Illinois, and Ohio, especially Geol. Surv. 

 111. Vol. 5. 



