486 E. O. ULRICH CORRELATION OF THE STRAND-LINE 



supported by the fact that included in the 16 species considered by the 

 writer of the paper mentioned 3 are identified with species known else- 

 where only in Cayugan or Helderbergian deposits. 



But why deny consideration to the 37 new species that really make up 

 the bulk of the fauna of the formation in question? Surely they have 

 some bearing on the problem. Why not determine the relative nearness 

 of their relations to their Niagaran allies on the one hand and to their 

 congeners in the Cayugan and Helderbergian faunas on the other? It 

 seems a careless procedure to ignore this additional and perhaps most 

 important means of attaining the truth. 



It happens that I know the fauna of the McKenzie formation rather 

 well. The new species in it are underrated at 37, the number reaching 

 at least 60. The Ostracoda alone make up more than one-half of this 

 number, and only 5 of these are at all closely allied to Niagaran species. 

 On the other hand, the same species, as well as all the other new Ostra- 

 coda of the McKenzie, are exceedingly like species found abundantly in 

 overlying Cayugan and Helderbergian formations. It is indeed fairly 

 questionable if more than half of the McKenzie Ostracoda are specifically 

 distinguishable from their later representatives. On the other hand, it 

 has been positively determined that at least 50 per cent of the species of 

 Ostracoda found in the overlying Wills Creek formation are practically 

 identical with McKenzie species. 



Judging, then, from the known distribution of the Silurian and Devo- 

 nian Ostracoda, counting the new species as well as the old, the fossils of 

 this class in the McKenzie formation are decidedly indicative of a post- 

 Niagaran age. 



Study of the other classes of fossils in this formation similarly led me 

 to the conviction that, taken as a whole, they resemble Cayugan and 

 Helderbergian species more than Niagaran. Whether this be so or not, 

 the fact remains that the bulk of the fauna as now known is distinct from 

 all known Niagaran faunas. This of itself indicates that it is of later 

 date, even though the differences are in large part due to the fact that 

 whereas most of the known Niagaran faunas in America are either of 

 southern or boreal origin, the Cayugan and early Helderbergian faunas 

 in the Appalachian region invaded from the Atlantic. 



The Lower Clinton fauna in the Appalachian region consists wholly of 

 Atlantic derivatives. The same is true also of the Appalachian upper or 

 Eochester Clinton fauna, except in central New York, where the Atlantic 

 fauna interfmgers with bryozoan faunas that invaded from the southwest. 

 In Maryland and Pennsylvania, therefore, the Eochester zone contains 



