THE DEVONIAN PERIOD. 473 



they are there preserved in beds which overlie beds containing Oriskany 

 forms. This means that they lived in South America at a time just 

 previous to or nearly identical with their appearance in the North 

 American fauna. That they were immigrants from the south is the 

 almost necessary inference. 



The advance of the cephalopods. — The mollusks were an interesting 

 factor of the Hamilton fauna. The new lo bate-sutured cephalopods, 

 the goniatites, which were represented by a single known species in 

 the previous fauna, now appeared in six species. One of these, Gonia- 

 tites vanuxemi (Fig. 210, c) reached the notable size of a foot in diam- 

 eter. The Nautilus regained the prominence it had lost since the 

 Niagara epoch and appeared in forms rivaling the large goniatites, 

 and some of them began to exhibit the node-like ornamental expan- 

 sions of the shell which became so conspicuous in the later Carbon- 

 iferous faunas. It seems to have replaced the more loosely coiled 

 Cyrtoceras and Gyroceras of the previous fauna. The straight forms 

 were present and were closely allied to those of the Onondaga epoch. 



The abundance of the pelecypods. — The silty bottoms favored the 

 pelecypods, and they developed in great abundance, the number of 

 recognized species approaching 200. The aviculicls of the obliquely 

 winged type were the most conspicuous and characteristic. A repre- 

 sentative form is illustrated in Fig. 210, e. A part of the pelecypods 

 were obviously derived from Onondagan predecessors, while others 

 were peculiarly Hamilton types, either immigrants, or forms so widely 

 different from Onondagan forms that their parentage is unrecognizable. 



The subordinate place of the gastropods. — Though fairly abundant, 

 the gastropods were quite inferior to the pelecypods in numbers. The 

 genera were almost entirely the same as in the Onondaga epoch, though 

 the species were largely new. The capulids were still abundant, but 

 by no means so plentiful as in the preceding faunas. The pteropods, 

 again on the ascendant, were represented by Conularia, tentaculites, 

 and a minute Styliola which largely formed certain thin beds of lime- 

 stone, as the tentaculites had previously done. 



Other features. — The Hamilton trilobites differed notably from 

 the Onondagan in being much inferior in numbers and much less orna- 

 mented. The dalmanites w T as present, but in relatively modest garb. 

 As an offset, the other crustaceans were more amply represented by 

 the phyllocarids (Fig. 210, a) and cirripeds (Fig. 210, b). Among the 



