Three Great Fossil Placoderms of Ohio. — Claypole. 95 
with a corresponding upper bone, as one blade of a pair of 
scissors slides over and past the other. When to this we add 
the fact that this jaw. so formidably armed and fitted out with 
weapons of destruction and defense, was two feet in length 
and evidently worked by a powerful set of muscles, we can in 
some degree realize the dominating position which its owner 
could and doubtless did assume among his contemporaries. I 
say "doubtless did," for at least one specimen is known which 
shows upon its surface the prints of the terrible fangs of Din- 
ichthys. 
Such a structure is almost or altogether unparalleled, not 
only among fishes, but in the whole animal kingdom. Indeed, 
it is not even a character of the genus, for the predecessor of 
D. Terrelli of the Cleveland Shale, the equally formidable D. 
I/ertzeri of the Huron, did not possess this terrible pair of 
shears. Its lateral teeth were of no extraordinary shape or 
size. So far, therefore, as we yet know, D. Terrelli is unique 
upon earth. Nature here attempted a new type of dentition 
which she never afterwards repeated. Did it fail in effective- 
ness as do many of the patents of man? Possibly so. At all 
events the pattern was abandoned, laid on the shelf and no 
more castings made. All later teeth were moulded on the 
pattern previously adopted. 
The second member of this trio is the still more gigantic 
Titanichlhys of Terrell, found in 1883. Surpassing Dinich- 
thys in length of jaw, at least as three to two, its defensive 
plates form the largest armour known. But it was inferior in 
massiveness. Its head-plates are thin and light when com- 
pared with those of its rival. It measured four feet and four 
inches in breadth, but with the exception of a few separate 
bones we know little of the body of the animal. Some of the 
jaws already found, though three feet in length, are Blender 
by the side of the massive bone of the former fish. The jaw 
or tooth in front slopes forward at an obtuse angle, and is 
therefore ill adapted for the purpose of closingwith a snap, as 
did the tremendous "steel trap" of Dinichthys. We may. there- 
fore, infer that its owner was less formidable than his more 
heavily armour-clad companion. 
The jaw, behind the front tooth, is hollowed out in a groove 
or furrow (alveolus) which Newberry suggested may have 
