530 GEOLOGY. 



by the presence with them, as far east as Arkansas, of Rhynchonelfa 

 eurekensis, a peculiar and distinctive Nevada species. 1 



The decline of the crinoids. — Either the invaders, or else changes 

 in the physical or organic conditions, influenced adversely the crinoids, 

 for, although they still formed a large factor in the assemblage, they 

 had fallen off much from their abundance in the Osage fauna, and 

 had undergone notable changes of form (Fig. 238, a-c). Less than 

 25 species of camerate crinoids are known in the entire Genevieve 

 fauna, as compared with upwards of 300 in the Osage. Among these 

 no Osage species persisted, and in the faunas younger than those of 

 the St. Louis limestone, not even an Osage genus. The inadunate 

 and flexible crinoids, however, did not show so remarkable a decline. 

 Among the crinoids some new and curious forms were developed, 

 among which may be mentioned Agassizocrinus (Fig. 238, a), Ptero- 

 tocrinus (Fig. 238, c), and Acrocrinus (Fig. 238, b). 



The culmination of the Mastoids. — The blastoids had their climax 

 here so far as numbers of individuals are concerned, although there 

 was much greater generic diversity in the Osage faunas. The leading 

 genus, Pentremites (Fig. 238, d), was so prolific in individuals in some 

 beds as to give rise to the name Pentremital limestone. A swift decline 

 seems to have followed this sudden climax, and the beautiful forms 

 disappeared for reasons quite unknown. 



The last of the known cystids, a species of the peculiar, horizon- 

 tally flattened, disk-like Agelacrinidce, a family which had persisted 

 from the Ordovician period, was a member of this fauna. 



Echinoids were present, among them an ample rotund form sug- 

 gestively named Melonites, and resembling the Oligoporus (Fig. 236, k) 

 of the Osage epoch. Although the echinoids had appeared much 

 earlier, it was in the Osage and Genevieve faunas that they for the 

 first time showed signs of the prominent development they were to 

 attain later. 



Partial recovery of the corals. — The corals seem to have profited 

 by the decline of their rivals, the crinoids, or by other conditions, for 

 they were more numerous than in the Osage fauna. A compound 

 coral, growing to great size, Lithostrotion canadense, became con- 

 spicuous in the St. Louis limestone at some localities. Aside from 

 this, however, the simple horn-shaped forms remained the most common. 



1 Williams: Am. Jour. Sci., Vol. XLIX, p. 94. 



