THE SKULL OF PALEORHINUS 149 
It scarcely seems wise under the circumstances to make any positive 
statement regarding the generic identity or separateness of these two 
specimens without comparison of all the material available, and this 
has not been possible. There are other nearly perfect skulls in the 
collection from the Popo Agie beds quite different from the present 
one which have not yet been studied. Dr. Williston is doubtful as to 
the distinction, but inclines toward the belief that they belong to the 
same genus. See his paper on this specimen previously quoted. 
It is chiefly because of the marked differences between the ilia of 
Rhytidodon carolinensis as figured by McGregor and of the specimen 
in hand, differences which are believed to be generic in value, that the 
specimen is considered as of another genus. ‘These differences may 
be briefly summarized here. They are: the absence of the trans- 
verse ridge from the outer face of the ium of Rhytidodon and its 
presence in that of Paleorhinus; the angularity of the processes in 
Rhytidodon contrasted with their roundness and broadness in Paleo- 
rhinus; the convex upper edge of the bone in Rhytidodon and its 
general concavity in Paleorhinus and the greater size and strength of 
the central process in Paleorhinus. 
A comparison of the skulls of the two genera is somewhat difficult 
owing to the incomplete condition of the skulls of Rhytidodon. ‘The 
distinction which impresses itself most strongly upon the attention is 
that between the proportions of the skulls. McGregor states (p. 59) 
that the prenarial portion of the skull of Rhytidodon must be two- 
thirds the entire length. In Paleorhinus, on the other hand, the 
pre- and postnarial portions are nearly equal—the prenarial portion 
is one inch the longer. 
Another distinction between the two genera is the long backward 
projection of the squamosal region over the quadrates in Rhytidodon, 
while in Paleorhinus the quadrates and bones associated with them 
extend almost as far backward as does the upper surface of the skull. 
Further, in Rhytidodon the external nares are situated over the 
internal and le opposite and above the middle of the antorbital 
vacuities. In Paleorhinus, however, the external nares lie far in front 
of the internal openings, which are situated opposite the middle of the 
antorbital vacuities. The external nares thus occupy a position well in 
advance of the antorbital vacuities. This difference is due to the 
