THE DEVONIAN PERIOD 581 



from which they were doubtless descended. Trilobites were present 

 in more than half a hundred species, a notable increase over the 

 number known in the same region in the preceding epoch. The 

 high degree of ornamentation of many of the species was a con- 

 spicuous feature. Barnacles were present, as were also annelids, 

 sponges, hydrozoans, bryozoans, and protozoans. All of these played 

 inconspicuous parts, though in the economy of the whole they were 

 doubtless not unimportant. 



Reviewing these features, it may be said that certain of the 

 forms were clearly the descendants of species that came into the 

 interior sea from the eastward as members of the Helderberg and 

 Oriskany faunas. At the same time, there were prominent elements 

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

 which seem, with equal clearness, to have come in from some other 

 source. A very characteristic development of these latter elements 

 is found along the Straits of Mackinac, while to the north, in the 

 James Bay basin, less than 300 miles away, there was a fauna of 

 similar aspect. This suggests a connection between these localities, 

 along which these new elements of the fauna migrated; but there is 

 no positive evidence of a former connection across the intervening 

 tract. It is known, however, that the formation has been, in part 

 at least, removed from this tract. On the whole the striking 

 features of the fauna seem to be most readily explained by sup- 

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

 the American or European continent, and that from this source 

 migration into the interior sea took place as the waters from both 

 the north and the south extended themselves upon the face of 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. 

 In further support of this conclusion, there is no evidence that any 

 of the Onondaga species supposed to have come from the north 

 found their way into South America, while, on the other hand, 

 certain of the Oriskany species lived there and certain of the Ham- 

 ilton species, which appear to have come in from the south a little 

 later, also lived in South America. 



1 This conclusion is not universally accepted. 



