THE MISSISSIPPIAN PERIOD. 527 



rently a colony of their common ancestors had established itself at 

 this locality, perhaps in late Devonian times, and, finding it free from 

 the hostile conditions prevalent in the region, flourished in isolation 

 for a period sufficiently long to differentiate species of its own, after 

 which it died out. 



As might be anticipated from the silty conditions, the pelecypods 

 were the most prominent feature of the fauna. Many of them were 

 of the same species as the previous occupants of the region in the closing 

 Devonian times, showing again the close relations of the two periods. 

 The other mollusks were relatively less abundant. Brachiopods 

 were numerous and resembled those of the contemporaneous Kinder- 

 hook and Osage faunas ; but they were, on the whole, rather less differ- 

 entiated from the ancestral Devonian forms, doubtless because the 

 change in their environment had been less notable. The other forms 

 were not very different from those of the more open sea to the 

 westward. 



The Waverly fauna was the direct descendant of the Devonian 

 faunas that had occupied the same ground and had changed but 

 slowly, as the environment remained nearly constant. It was modified 

 by some immigration of the Kinderhook and Osage types, and this, 

 combined with its own conservative progress, caused it to take on 

 gradually a Mississippian aspect, while it retained many Devonian 

 characteristics. It is a good illustration of a lingering fauna where 

 the conditions are conservative, though not so good an illustration 

 as the next fauna. 



The Great Basin Fauna. 



It will be recalled that in Devonian times the Great Basin was a 

 province by itself, in which a slow faunal development of provincial 

 aspect, only slightly modified by foreign contributions, took place. 

 At the horizon rather arbitrarily fixed upon as the upper limit of the 

 Devonian system, there appears to have been no faunal break. It is 

 necessary to speak with some reservation, for the region has not yet 

 been thoroughly worked, paleontologically. The transition to the 

 Mississippian facies seems to have taken place very gradually, through 

 the progressive evolution of some forms, the elimination of others, 

 and the immigration of a few from westerly sources, but, until after 

 the Osage epoch, rarely from easterly sources, so far as now known. 



