PEOFESSOE OWEN ON THE DICTNODONT EEPTILIA. 
457 
rising and the slightly elevated orbital borders. There is no trace of the pair of tubercles 
which, in Ptycliognathus declivis*, stand transversely on the frontal platform, in the line 
bisecting the middle of the orbits : they have not been chipped away in clearing off the 
matrix, either in the present skull or in that of Ptycliognathus latirostris\. The median 
rising at the frontal suture is continued backward over the part where it divides to go to 
each tubercle, leaving an intermediate shallow depression, in Ptycliognathus declivis. The 
frontal tubercles may be, therefore, reckoned among the cranial characters distinguishing 
Ptycliognathus declivis from Ptycliognathus latirostris, and from the skull of the species 
under description. In this, as in the previously defined species of Ptycliognathus, the 
vertex or upper surface of the skull is bounded anteriorly by a low ridge (fig. 2,f,f), ex- 
tending transversely with a slight curve, convex forward, from the fore part of one super- 
orbital border to the other. The ridge is formed by the nasals (fig. 2, is) at the middle, 
and by the prefrontals {ii. u) at the sides. The latter are strong, large, angular bones 
(figs. 1, 2 & 3, u), dividing by a prominent tuberosity the fore from the upper borders of 
the orbit : the frontal surface of the prefrontal is divided by the before-described ridge 
from the facial surface ; both are slightly concave, the latter of greater extent, reaching 
almost to the hinder angle of the external nostril (figs. 1 & 3, n), and articulating with 
the nasal (ib. is) above, and with the lacrymal (ib. 73) below. The nasals (is), divided by 
a median suture, bend abruptly, at the prefronto-nasal ridge, from the upper to the fore 
or facial part of the skull, at an open angle. They are united for about an inch as they 
slope upon the face (fig. 2), and then diverge to form the upper border of the nostrils, 
receiving at the angles, so formed, the upper and hinder ends of the coalesced premaxil- 
laries, ib. 22 . The premaxillaries continue the facial line, begun by the nasals, straight 
to the upper margin of the mouth, which is directed forward. There is a slightly 
elevated line along the place of the obliterated medial suture (fig. 2, ni), parallel with 
which is a pair of similar linear ridges (fig. 2,y>,p), dividing the median from the lateral 
surfaces of the premaxillary : these surfaces have almost the same breadth, except that 
the lateral surface (fig. 1, 22 ) increases near the oral border; it is narrowest where it 
forms the fore part of the nostril, n. 
The maxillary (figs. 1 & 3, 21 ) is chiefly remarkable, as in the rest of the genus, for 
the strong ridge-like prominence (r r) of the socket of the long and large canine tusk, c. 
Between this and the outer premaxillary ridge (fig. the sides of the face are slightly 
sinuous, the convexity above gradually changing to a deeper concavity below, in the 
transverse or vertical direction ; lengthwise the sides of the face are slightly concave as 
they converge towards the mouth; except at the canine alveoli, which are a little 
convex lengthwise at their fore part. Above, the maxillary bounds the lower part of the 
nostril, and there unites with the premaxillary (22) and lacrymal (73) ; below, it is con- 
tinued from the canine-socket inward and dov/nward, forming the edentulous sectorial 
border which meets the corresponding part of that border of the mandible (32), the fore 
part of which, in advance of the maxillary, is strongly bent upward (32') to meet the eden- 
* Quarterly Journal of the Geological Society, vol, xvi. p. 50, pi. 1. fig. 3. f Itid. p. 51. 
