162 PALEOZOIC FISHES OF NORTH AMERICA. 
The dental plates of the upper jaw are tabular and consist of very dense 
tissue; they probably formed several pairs on opposite sides of the median 
line; the largest is somewhat triangular in outline, with a concave tritu- 
rating surface and vertical sides, apparently for co-adaptation to other teeth 
of the set. Others of these paiate teeth are shorter and broader, with one 
margin concave, apparently for fitting the rounded posterior end of the larger 
teeth just described. The bases of these teeth or dental plates are flat, and 
they were apparently attached to the roof of the mouth. 
It is probable that we have not yet obtained all the elements in the 
dentition of this fish, and the parts yet discovered are so peculiar and anoma- 
lous as to make it difficult to co-ordinate them satisfactorily with any others 
hitherto known. The flattened tabular dental plates which I have supposed 
formed the roof of the mouth have a general resemblance in form and text- 
ure to those of Chimera, and it is evident are their analogues and function- 
ally their representatives; still the teeth of the under jaw found with these, 
and exhibiting the same microscopic structure, differ widely from any por- 
tion of the dentition of Chimeeroid fishes and show a greater resemblance to 
the dental plates of the Dipterine Ganoids. They evidently formed pairs, 
for we have the corresponding teeth of the right and left sides, and though 
wanting the radiating ridges of the teeth of Ctenodonts, they seem to have 
occupied corresponding positions. The strong vertical supports on which 
they are mounted correspond with the splenial bones that carry the inferior 
dental plates of Ctenodus, except that they are flattened vertically instead 
of horizontally, and probably represent more of the mandible. 
The resemblance of the teeth which I have supposed formed the roof 
of the mouth to those of Ceratodus will strike any one who examines them, 
and no closer analogy suggests itself in the whole range of ichthyic denti- 
tion. There is, however, this marked difference, that while in Ceratodus 
there is only one pair of dentary plates borne on the palato-pterygoid 
bones, in Mylostoma there were certainly several pairs of pavement teeth in 
the roof of the mouth. The spatulate bones which form the supports of 
the principal dental plates of the lower jaw evidently represent the thin, 
flattened, smooth, and once buried posterior end of the dentary bone in all 
of the Dinichthide ; and, taken by itself, each of these dental plates with its 
