PEOFESSOE OWEN ON THE DICTNODONT EEPTILIA. 
459 
tically and transversely. The longitudinal ridge on the outside of the dentary, parallels 
that formed by the canine alveolus of the maxillary ; and the alveolar borders in both 
incline inwards, to meet at the oral margin behind the great tusks. The mandibular 
ridge resembles the maxillary one in Oudenodon ; and one is led to speculate whether in 
that genus the maxillary ridge may have related to a rudimentary upper canine, and the 
mandibular ridge to that of a lower one, in the embryo. In both PtycJiognathus and 
Oudenodon, however, these parallel ridges in the upper and lower jaws, with the inward 
convergence of the alveolar plates, recall the structure of those parts in Scelidosaurus *, 
and which is indicated by the mandibles to have existed, in a certain degree, in Hylmo- 
saurus and Iguanodon. 
The fossil skull above described was obtained from a greenish sandstone of the Rhe- 
nosterberg. As it manifests specific distinctive characters, I propose to refer it to a 
species of PtycJiognathus, under the name of the young Prince by whom it has been 
made available in advancing our knowledge of South African fossils. 
The Ptychognathus Alfredi, like Ptychognathus latirostris, differs from Pt. declivis in 
the more circular orbits, the absence of the frontal tubercles, the right angle at which 
the occiput and vertex meet, and the greater depth and thickness of the facial part in 
proportion to its length. 
But on comparing Ptychognathus Alfredi with Ptychognathus latirostris, the facial 
part of the skull presents less breadth in proportion to its length and depth, and the 
lower jaw is narrower in proportion to its length ; the paroccipital is also relatively 
narrower. 
The occipital region in Ptychognathus declivis (Plate XXI. fig. 2) shows nearly the 
whole extent of the masto-tympanic pedicle, and well exemplifies the singular breadth of 
the hinder surface of the cranium in these dicynodont reptiles. It also shows the pre- 
condyloid foramina, and the small vacuity between the exoccipital and masto-tympanic 
bones. The side view of the skull of Ptychognathus latirostris (Plate XXI. fig. 1) well 
exemplifies the ridged structure of the jaws and the composition of the mandible. 
In the corresponding view of the skull of Ptychognathus Alfredi (Plate XX. fig. 3) 
is shown the extent of the pulp-cavity of the canine (c), closely conforming to that in 
the instances of ever-growing tusks in the mammalian class. As in former specimens of 
true Picynodon, the section made to show the base of the tusk exhibits no trace of any 
germ of a successional tooth. The tusk curves forward, downward, and slightly inward. 
The points of both tusks have been broken away. 
Picynodon tigriceps, Owen. 
The larger fossil, obtained by H.RH. Prince Alfred from the Karoo-beds, in the 
district of Graaf Eeinet, is a skull of a true Picynodon, equalling in size that of the 
Picynodon tigriceps^ . It has been obliquely crushed, but under circumstances of sur- 
* “ Monograph on British Eossh Eeptiles,” Palaeontological Society’s volume for 1860, p. 12, pis. 4& 5. 
t Transactions of the G-eological Society, 2nd Series, vol. vii. p. 233. 
