FISHES OF THE CARBONIFEROUS SYSTEM. 163 
support may be regarded as the dentary bone of the mandible. But the 
margjns of these triturating plates are smoothed vertically or beveled in 
such a way as to indicate co-adaptation to other plates. We have seen that 
one pair, described above, probably fitted on to the anterior extremities of 
these larger plates, and there may have been others at the sides or behind. 
Such a division of the dentary bone into distinct parts is not altogether 
without precedent, for in Dendrodus and Rhizodus the dentary bones are 
segmented, each piece carrying one or more of the great, conical, socketed 
teeth which constitute the formidable dentition of these fishes. 
The modifications of structure in the mouths of fishes recent and ex- 
tinct to give support to pavement or crushing teeth are very varied, as will 
be seen by reference to the dentition of the Pycnodonts among fossil fishes 
and to the drum, sheepshead, ete., of our living fauna. Among Elasmo- 
branchs the Cestracionts and Rays present an almost infinite variety in the 
form and arrangement of the teeth adapted to crushing mollusks and crusta- 
ceans. But if we are correct in considering Mylostoma as a member of the 
family of the Dinichthidee, we have in it an example of extreme specialization 
in another direction, but no less marked and surprising than in Dinichthys 
and Diplognathus. ‘Ina former notice of this genus I have compared its 
dentition with that of Dipterus, Paledaphus, Ctenodus, and Ceratodus, and the 
points of resemblance are sufficient to justify the inference that they were 
all related. ; 
In regard to the cranium and external defenses of the body in Mylostoma 
we are yet in ignorance, although the number of dental plates collected by 
Mr. Terrell is already considerable. I have elsewhere described a portion 
of the body plates of a large Placoderm, Glyptaspis, found in the Cleveland 
shale, with which no jaws and teeth have yet been comnected, and have 
suggested that either the jaws of Diplognathus or the teeth of Mylostoma 
might have belonged to this fish. It is hardly probable that, while living 
in a community so generally armor-clad as the Dinichthide were, and with 
reason, that either Mylostoma or Diplognathus was without some protection 
of this kind; and though neither of these genera could have contended suc- 
cessfully with the formidable armament of Dinichthys Terrelli, they would 
have been less entirely at his mercy if the vital portions of the body were 
