THE OLD RED SANDSTONE. 153 
Upper Old Red Sandstone, I shall have to draw mostly from 
single specimens. But the evidence may be equally sound 
so far as it goes. 
The difference between the superior and inferior groups 
of the system which first strikes an observer, is a difference 
in the size of the fossils of which these groups are composed. 
The characteristic organisms of the Upper Old Red Sand- 
stone are of much greater bulk than those of the Lower, 
which seem to have been characterized by a mediocrity of size 
throughout the entire extent of the formation. The largest 
tchthyolites of the group do not seem to have much exceeded 
two feet or two feet and a half in length; its smaller average 
from an inch to three inches. A jaw in the possession of Dr. 
Traill—that of an Orkney species of Platygnathus, and by 
much the largest in his collection — does not exceed in bulk 
the jaw of a full-grown coal-fish or cod ; his largest Coccosteus 
must have been a considerably smaller fish than an ordinary- 
sized turbot; the largest ichthyolite found by the writer was 
a Diplopterus, of, however, smaller dimensions than the ich- 
thyolite to which the jaw in the possession of Dr. Trail] must 
have belonged; the remains of another Diplopterus from 
Gamrie, the most massy yet discovered in that locality, seem 
to have composed the upper parts of an individual about two 
feet and a half in length. The fish, in short, of the lower 
ocean of the Old Red Sandstone —and I can speak of it 
throughout an area which comprises Orkney and Inverness, 
Cromarty, and Gamrie, and which must have included about 
ten thousand square miles—ranged in size between the 
stickleback and the cod; whereas some of the fish of its 
upper ocean were covered by scales as large as oyster-shells, 
and armed with teeth that rivalled in bulk those of the croco- 
dile. They must have been fish on an immensely larger 
