cup f< 

 corals 



LIFE 417 



ir ancestors had been living for a long time in the region where 

 ey originated, probably somewhere in the north. 



Another significant feature of the Onondaga fauna is the pro- 

 fusion of corals. From the rapids of the Ohio at Louisville, more 

 than 200 species have been collected, embracing both the simple 

 p form (a, Fig. 365) and the compound type. Some of the cup 

 attained a length of 18 inches and a diameter of 3, but the 

 range in size was great. The reef-building habit attained greater 

 development than in Silurian times, the reef at the rapids of the 

 Ohio being the most famous example. Crinoids were rather few, 

 but they do not appear to have lost their vitality, for they were 

 abundant later. Large Brachiopods and cephalopods were plentiful. 

 It will be remembered that in the primitive types of the cephalo- 

 pods, the septa of the shells were plane or symmetrically curved, and 

 that their juncture with the outer shell was a simple curve. In the 

 Onondaga epoch, one form had septa which were bent abruptly, and 

 suture lines which were lobed (i, Fig. 365). This was the first 

 notable step in a remarkable series of crumplings of the septa which 

 eveloped later. Gastropods similar to those of the earlier Devo- 



n faunas were present, and the spines of the shells had now become 

 pronounced in one group of them, perhaps signifying the necessity 

 of defense against the abundant fishes and cephalopods. Pelecy- 

 pods were abundant, many of them descended, no doubt, from 

 Helderberg and Oriskany ancestors. Trilobites were present in 

 more than half a hundred species, some of them being highly orna- 

 mented. 



It seems clear that some of the species were descendants from 

 the Helderberg and Oriskany faunas. Other prominent elements 

 of the fauna, particularly the fish, cephalopods, and corals, seem, 

 with equal clearness, to have come in from some other source. 

 The striking features of the fauna seem to be explained by supposing 

 that there was a generating tract to the north, 1 either on the Ameri- 

 can or European continent, and that from this source migration 

 into the interior sea of North America took place as the waters from 

 the north extended themselves over the continent. As the result 

 of the invasion, some part of the Oriskany fauna which already 

 occupied the interior sea was driven out or destroyed, while the 

 rest intermingled with the northern invaders. 



1 This conclusion is not universally accepted. See Schuchert, Bull. Geol. Soc. 

 Amer., Vol. XX. 





