PEOFESSOE OWEN ON THE DICTNODONT EEPTILIA. 
461 
meter ; in its length or extent of ossification it exceeds that in the Chelonia, and more 
resembles that in the Crocodilia ; at its base there arise some thin rays of ossification, 
which ascend to unite with a similarly attenuated lamelliform base of the orbitosphenoid. 
I infer that there was persistent cartilage in this part of the skull. The orbitosphe- 
noid {ib. lo) becomes thicker as it ascends to unite with the under part of the hinder third 
of the frontal, and with part of the parietal. It is perforated by a foramen opticum of 
elliptical form, 6 lines in long diameter. 
Below and anterior to the presphenoid is seen a small part of the vomer [ih. 13), where 
it expands laterally to join the palatine {ib. 20) and pterygoid, ib. 24. The pterygoid, 
about 5 inches in length, contracts as it extends backward, bounds above or mesially the 
outer part of a long elliptical foramen (posterior nostril, ib. n), and then bends downward 
and outward to join the basisphenoid and abut against the lower end of the tympanic. 
There appears to have been a part of the interorbital space unossified, about 3 inches 
in length, 1^ inch broad below, but suddenly contracting to a width of ^ an inch at the 
upper part. 
The exposed bottom of the right socket of the canine tusk (ib. fig. 1, c) shows the 
similarly exposed pulp-cavity of the beginning of that tusk, which measures inch 
across. The walls of the tusk increase in thickness to about 3 lines in an extent of 
inch. A greater extent of the left socket (Plate XXII. fig. 2) is preserved, showing 
the concentrically lamelliform structure of the base of that tusk. 
Sufficient of the mandible is preserved to demonstrate its characteristic dicynodontal 
composition. The alveolar border of the dentary element is toothless. The ramus 
rapidly augments in depth towards and at the symphysis, where a portion is broken 
away. The posterior part of the dentary {ib. fig. 1, 32) is notched or bifurcates to form 
an oblong vacuity at the middle of the mandibular ramus. Traces of a long surangular 
{ib. fig. 1, 3 o), which developesno coronoid process, of a broader or deeper angular {ib. 31), 
and also of a splenial {ib. 29), are discernible amongst the elements of the lower jaw. 
As a whole, the present instructive specimen exemplifies the near equality in size of 
some of the strange extinct two-tusked reptiles of South Africa to the existing Walrus; 
it also shows that in the structure of the bony palate the Dicynodon combines, as in other 
parts of the skull, crocodilian with chelonian and lacertian characters. 
Explanation op the Plates. 
PLATE XIX. 
Fig. 1. Side view of skull of PtycJiognathus Alfredi'. nat. size. 
Fig. 2. Upper view of the same skull ; nat. size. 
3 E 
MDCCCLXII. 
