148 SIR RICHARD OWEN ON THE SKULL AND DENTITION 
cavity, represented by the matrix exposed by the breaking away 
of the occipital part of the skull. 
The thin plates of bone bounding the postorbital part of the 
cranium may include parts of the parietals, 7, and alisphenoids. 
There is no trace of an ectopterygoid. The loss of the occiput is to be 
regretted; but the mammalian characters in the preserved parts of the 
skull dispense with the confirmatory conclusion of double condyles, 
which, however, are common to Batrachians as well as Mammals. 
I next pass to the dentition of Tritylodon. Its condition in 
the fossil indicates the following formula :—z =, m 32. The 
grinding-surfaces of the upper teeth bespeak the number of the 
lower ones; but, in the case of the incisors, a single modified 
pair might have been opposed, after a leporine fashion, to the 
two pairs above. As in the Leporide also, the six upper may 
have been opposed by five lower grinders; but in Tritylodon the 
upper grinders are implanted by long fangs, two of which are 
external: the molars of Leporide are rootless. 
As before remarked, the crowns of the front and chief incisors 
are broken off near their sockets. Sufficient remains to show a thin 
coat of enamel upon their front and sides: the exposed dentine be- 
hind is scored by vertical striz ; any cement which may have covered 
the strie has perished. The dentine has the usual mammalian com- 
pactness. The size, sectional shape, and relative position of these 
incisors are given in Pl. VI. figs. 2,3, &71. The exposed part of the 
pulp-cavity, fig. 2, p, is of a size suggestive of these incisors having 
enjoyed uninterrupted growth; removal of part of the outer wall 
of the socket of the left incisor showed the body of the tooth to be 
continued without diminution through the premaxillary into the 
maxillary, as ina scalpriform incisor of continuous growth. Behind 
each of the incisors, with an interval of 2 millims., is the socket of 
a smaller premaxillary tooth, fig. 2, 2 2, of which the remains of a thin 
wall only of the base of the tooth can be traced, enclosing a pulp- 
cavity relatively larger than in the front incisor, but, as in it, filled 
with the reddish matrix. This difference of size of pulp is due to 
the fracture of the implanted root, and is also suggestive of con- 
tinuous growth of the tooth. After the ridged diastema without 
trace of socket, we come upon a similar mutilated condition of the 
tooth (fig. 2, m1) which is the foremost of a series of six con- 
tiguous grinders. The crown of this tooth is subtriangular, as 
as seen on this fractured surface, the base turned backward and 
in contact with the next molar, m2. This tooth is the foremost 
of five similar molars, to which it is very little inferior in size. It 
presents the same subquadrate figure, and shows the external notch 
partially bisecting the outer side of the tooth (fig. 5); its crown 
being broken away at the level of the alveolus, the character of 
the part lost is given by the succeeding teeth. In these the crown 
is impressed by a pair of antero-posterior grooves, dividing the 
grinding-surface into three similarly disposed ridges, and each ridge 
is subdivided by cross notches into tubercles (fig. 7). Of these, there 
are, in the second to the fourth molar inclusive, four tubercles on 
