132 PALEOZOIC FISHES OF NORTH AMERICA. 
has disappeared. This may have consisted of one or several bony wedges, 
or what is more likely, was a coating of horn, as in the turtles. Similar 
rods to the mandibles of Titanichthys Agassizii have been found beneath the 
head of Hugh Miller's Asterolepis (Homostius, Pander), a Placoderm closely 
allied to Dinichthys. These rods would be useless as organs of prehension 
or mastication, and the thought suggested itself to me, on seeing a fine series 
of heads in the hands of Dr. Traquair at Edinburgh, that they must have 
been sheathed in horn. The jaws of Titanichthys nowhere show the con- 
densed tissue and acute worn edges of the mandibles of Dinichthys, and it 
is evident that such a bony dentition for cutting or piercing did not exist 
in the former genus. The texture of the bone where the teeth or cutting 
edges should be is porous, and shows no evidence of wear or use; hence it 
seems inevitable that it must have been sheathed with some denser material, 
that encountered the wear and violence to which dental organs are exposed. 
Nothing corresponding to the plastron of Dinichthys and Coccosteus, 
composed of five distinct plates, has been found associated with the other 
bones of Titanichthys, but instead a single large triangular plate, which may 
have been its representative; at least its symmetry indicates that it was 
placed on the median line, and, since the back was covered with a dorsome- 
dian plate, we are compelled to locate it on the under side of the body. 
The affinities of Dintchthys and Coccosteus have been referred to in my 
description of Dinichthys in the Paleontology of Ohio, and it is certain that 
Titanichthys, Asterolepis (Homostius of Pander), Heterostius, and Coccosteus 
form a natural and closely assimilated group. The jaws of Coccosteus are 
imperfectly shown by Agassiz and Pander, and there was perhaps some 
variation of form in the different species; those they describe being narrow, 
flattened rods two or three inches long, bearing denticles on the upper 
margin near the anterior extremity. But I found in the British Museum a 
number of jaws of Coccosteus discovered since Hugh Miller, Agassiz, and 
Pander wrote about these fishes, in which the form is essentially that of 
Dinichthys, viz: the anterior extremity is turned up and forms a prominent 
denticle, and the whole organ is only a miniature copy of the mandible of 
Dinichthys Hertzeri. The difference in size is, however, very striking; one 
being three inches long, the other two feet. 
