/'jijic,- Devonian Placoderms of <>hit>. — Claypole. 355 
the previous calcareous mud and at the same time cooling 
down the Corniferous sea, and so rendering it less tit for tin- 
growth of corals. Onychodus and its companions were fishes 
of the open clear sea and probably left the region when the 
waters no longer suited them, so that for the ancestor of the 
Upper Devonian fishes we must seek some area where conge- 
nial conditions existed, that is to say, some Corniferous shore 
lines. But such shore lines are yet scarcely known in eastern 
North America, and have apparently been very largely de- 
stroyed. 
Here then is the first gap in our history. One volume at 
least is missing, and from Ohio we learn nothing more. Our 
state record is a blank touching vertebrate life before the days 
of the Lower Devonian limestone. Fishes appear in the Ohio 
waters fully developed, and for the earlier stages of their 
development we must look elsewhere. 
Many facts which cannot be detailed here, as they would 
unduly lengthen this paper and divert our attention from 
the main issue, lead to the belief that very great geographical 
changes took place in the Ohio region during the Devonian 
era. Especially about the middle of that era do we find evi- 
dence of such changes. Prof. H. S. Williams drew attention 
to one phase of this subject in his address to the geological 
section of the American Association for the Advancement of 
Science at Rochester in 1892. Another phase is indicated by 
the present topic. 'During the early Devonian era facts ren- 
der it probable that the sea of western North America and the 
Appalachian basin were separated by a land barrier. The 
different aspect of the two faunas does not admit a free and 
open waterway from one area to the other. But about the 
middle of the era there is an intrusion of new forms into t ln- 
latter which have apparently immigrated from the former. 
The facies of these strange species is not Appalachian, but 
rather European and induces the belief that an ocean ex 
tended over the northwest and thence to Europe, and that this 
ocean had been the previous home and origin of the intruding 
species. Now on this view it would be easy to refer the sud- 
den appearance of the dinichthyids to the same cause and. 
though the data regarding vertebrates are yet exceedingly 
slender, yet the presence of D. canadensis in Manitoba and of 
