464 GEOLOGY. 



the corals as organisms as in their great luxuriance and relative promi- 

 nence. The reef -building habit which had appeared in the Niagara 

 epoch reached a fuller expression at this time, the reef at the rapids 

 of the Ohio being the most famous example. This luxuriance of the 

 corals implies clear seas abundantly enriched with floating organic 

 matter. This seems in turn to imply that the higher lands and steeper 

 slopes which had perhaps given a notable inwash of sand and silt dur- 

 ing the two previous epochs had given place to lower land, doubtless 

 densely clothed with vegetation — the result, very likely, of an approach 

 to base-leveling. 



The unequal development of the crinoids. — That the profusion of 

 corals, however, did not find its sole explanation in clear genial waters 

 rich in food may be inferred from the relative poverty of the same sea 

 in crinoids which affect like conditions. In the Niagara epoch the 

 crinoids and the corals flourished together in luxuriance. In the 

 Onondaga, the corals were phenomenally prolific, while the crinoids 

 were, with local exceptions, rather scant. This is the more remark- 

 able in view of the subsequent, as well as previous, abundance of the 

 crinoids. That the physical conditions were not uncongenial seems 

 to be shown by the fact that the individuals present were usually large 

 and bore no signs of repressive environment, but rather the opposite. 

 The repressive conditions are possibly to be found in some untoward 

 feature of the transition from the Niagara epoch, which gave prece- 

 dence to their competitors, the corals, an advantage which they held 

 throughout the epoch to the measurable exclusion of the crinoids, 

 or else in some undeterminable organic agency. 



Cystoids have not been found. They were far down their declining 

 curve to be sure, but were not yet extinct, for they appear in the fol- 

 lowing period. The blastoids were represented (Fig. 208, /), but the 

 other echinoderms, like the cystoids, have not been found, and these 

 other branches were yet in their ascending stages. There was doubt- 

 less something in the conditions originating the Onondaga fauna that 

 was adverse to the whole order, while favorable to the corals. 



The changes in the brachiopods. — The ubiquitous brachiopods, as 

 usual, formed a large constituent of the fauna, both individual and 

 specific. Many were large and gave evidence of congenial conditions. 

 The spirifers (Fig. 208, d) and the stropheodonts (Fig. 208, c) were the 

 most characteristic types. The majority of the spirifers, however, 

 had a retrogressive look, in that the fold and sinus were generally 



