46 PALEOZOIC FISHES OF NORTH AMERICA. 
the opposite tooth; in others it was broader, and was fitted for crushing 
mollusks or other food. Internal structure of teeth cancellated; the trit- 
urating surface being roughened by the extremities of calcigerous tubes. 
The number of teeth and details of dentition are not yet known, but we 
may infer from their shape that they were placed at the anterior extremities 
of the head to form a kind of rostrum or beak, much as the dental plates of 
Chimera are placed. As none of the margins show marks of contact with 
other teeth, we may conclude there were but four having this form. 
The internal structure of these teeth varies somewhat in the different 
species, adapting each to its functions. In R. secans the central portion is 
cancellated by interwoven calcigerous tubes, while the surfaces are com- 
posed of tissue almost as dense as enamel, so that the cutting edges pro- 
duced by the friction of opposing teeth on each other were kept constantly 
sharp and effective. In R. frangens, however, in which the upper edge is 
thick and presents a broad triturating surface to the opposite tooth, the den- 
tal tissue is cancellated throughout, affording by use that peculiar rough- 
‘ened surface seen on the teeth of Cestracionts (Psammodus, ete.). 
Speaking of the zoological relations of Rhynchodus in the article referred 
to I used the following language : 
In regard to the affinities of Rhynchodus, it seems to me we have no good reason 
to doubt that they form the dentition of Chimeeroid fishes, and that we have in them 
evidence of the existence on the globe of the Holocephali at a period long anterior to 
the date of the strata in which their oldest remains have hitherto been found. As has 
been stated in the general review of our fossil fishes, the Chimzroids of our present 
seas (Chimera and Callorhynchus) are the remnants of an order of cartilaginous fishes 
which once held a much more important place than now in the fauna of the globe. In 
Europe the remains of the teeth of Chimeroids have been discovered in Tertiary, 
Cretaceous, and Jurassic strata, but none in older formations, if we except the some- 
what anomalous Ptyctodus found by Pander in the Devonian of Russia! * * * The 
affinities of Ptyctodus may be somewhat doubtful, although I have been inclined to 
consider the teeth described under this name as probably the dentition of some Chi- 
meroid fishes. 
Whatever may be thought of the relations of Ptyctodus, those who are familiar 
with the fossil Chimzroids described by Sir Philip Egerton will probably not hesitate 
to group Rhynchodus with them. 
Although the Holocephali have heretofore been supposed to be limited in their 
downward range by the Jurassic formation, since we have evidence that our living 
1 Ueber die Ctenodipterinen des Deyonischen Systems, p. 48, pl. 8. 
