ORINOIDS. 145 



specific existence more protracted, than the stratigraphy of 

 any one place would indicate. And thus certain forms would 

 become extinct in one region, and be completely replaced by 

 very different species; while in distant localities the migratory 

 forms would flourish in all their wonted vigor. 



Echinodermatous life during the Lower Carboniferous 

 was pre-eminently crinoidal and blastoidal — the former greatly 

 predominating in the earlier part, and the latter conspicuously 

 present in the later portion of the period. So marked is the 

 contrast between the faunal features of the middle and upper 

 portions of the Lower Carboniferous, that some writers have 

 suggested that the Burlington and Keokuk deposits could 

 very appropriately be called the " crinoidal" limestone; while 

 the St. Louis and Kaskaskia are manifestly a "blastoidal" 

 division. 



In the subjoined synoptical table* are arranged the prin- 

 cipal Carboniferous genera of the Crinoidea, and their distri- 

 bution through Paleozoic time. Inasmuch as the synonymy 

 of the species has been worked out more carefully and more 

 accurately than in any other group of fossils, the table is 

 especially reliable for the consideration of problems of distri- 

 bution during geologic times. The figures in the various col- 

 umns refer to the number of species in each genus at present 

 known from the respective beds. 



The genera enumerated in the accompanying synoptical 

 table, while characteristically Carboniferous, are very un- 

 equally distributed in time. In nearly every instance each 

 genus exhibits : ( 1 ) a gradual expansion after its first appear- 

 ance, shown by the differentiation of the species occurring in 

 each epoch; (2) a culmination, marked not only by a large 

 number of species and a great numerical increase of individ- 

 uals, but also by a remarkable development and specialization 

 of the various structural characters, and by a more or less 

 wide distribution in space ; and (3) a decrease in the number 

 of species, and a very apparent decline in physical energy, 



*The abbreviations are: L.. S.— Lower Silurian; U. S.— Upper Silurian; D. — 

 Devonian; W.— Kinderhooij; L. B —Lower Burlington; U. B.— Burlington; K.— Keo- 

 kuk; L — St. Louis; C— Kaskaskia; M —Coal Measures . 



