MANDIBLE STRUCTURE IN AMPHIBIANS AND REPTILES 627 
postsplenial, or, if preferred, the postopercular. The presence of 
this element has been definitely confirmed in Diplocaulus by 
Mr. Herman Douthitt, Fellow in Paleontology of the University 
of Chicago, who will shortly publish figures and descriptions of the 
cranium and mandible in this genus. 
A bone corresponding to that which I call the splenial has been 
determined in several stegocephalian mandibles; but under a 
misapprehension of the true relations of the bones posteriorly it 
has been called the infradentary, after a corresponding element 
in certain fishes. There can remain no doubt that it is identical 
with the bone now known to be characteristic of all primitive 
reptiles, as I have shown, which is certainly the splenial. It is 
also quite evident that both these bones, the splenial and post- 
splenial, correspond to the single splenial in the cotylosaurs and 
the crocodiles. Were we dealing with modern reptiles only, 
we should be justified in calling the posterior element the true 
splenial and the anterior one the infradentary. But we cannot 
conceive of such an anterior development of the posterior bone in 
Labidosaurus and Dimetrodon as I have shown to be the case. It 
is therefore practically certain that it is the posterior bone which 
has disappeared in the reptiles. Has it fused with the anterior 
bone? Or has it entirely disappeared? Whatever may be the 
case, it is quite evident that the bone requires a new name, which 
I have given it. 
I may add that the sutures shown in the figures of Trimeror- 
hachis are based upon a prolonged study of more than forty different 
mandibles of this genus. The sutures separating the articular from 
the surangular cannot be distinguished in any one of the forty 
specimens; that between the prearticular and the coronoid is less 
certain than the others, in its full extent. 
