;i_l The American Geologist. N,,v " '• ri " :; 
the supposition that, if a vertebrate fauna is ever found in the 
lower Devonian of the eastern Appalachian gulf, it will prove 
e different from that which is already known in < )hio 
While the species named above, with others, inhabited the 
, the open sea was possessed by another fauna. Indica- 
tions are not lacking of the ■ nee of the vertebrate fauna 
of the shales as has been demonstrated of the invertebrat< 
is far from improbable that the great placoderms of ( >hio are 
the descendants of others that held the muddy eastern shores 
iA the gulf in Corniferous days. Dinichthys lincolni proves the 
existence of that genus in New York in the Marcellus sub- 
period and is the more significant when we consider the almost 
total lack of fish fossils from the Hamilton group. 
On this view the great placoderms must have migi 
westward in the gulf a.s the condition of the bottom became 
more favorable and congenial to them (in consequence of the 
extension of the mechanical deposit poured into the northeast- 
ern area through the New York estuary) supplanting the pre- 
vious occupants of the ground either by displacement or de- 
struction. To the possible objection that no fossils of the kind 
are reported in the eastern Corniferous, the answer is obvious. 
Xo shore deposits of that age are known there. It is not easy 
to realize what must have been the extension of the Corniferous 
to the eastward. Ids beds have been so completely destroyed 
that the Corniferous limestone retains its purity in most places 
quite to its outcrop. But that there was a margin, and that 
mechanical sediments once existed there, may be assumed as 
axiomatic and in these sediments were probably buried the 
ancestors of the vertebrate fauna of the great shales of Ohio. 
The degree to which the eastern edges of the pakeozoic 
strata of Appalachia have been destroyed may be in some meas- 
ure realized upon consideration of the details given elsewhere 
by the writer, f which show that 153 miles of strata have ac- 
tually been shortened into 65 miles, thus compressing the sur- 
face and reducing its length along the line of section, by 88 
miles. Such a reduction- (allowing liberally for possible error) 
* Undetermined plates of placoderms exist in the Devonian of Iowa, 
according to Calvin. 
+ Pennsylvania before and after the elevation of the Appalachian moun- 
tains. Amer. Nat., March, 1X85. 
