AND CLASSIFICATION OF THE FOSSIL REPTILIA. 
ooo 
bones. A deep groove separates these two bones ; and, on the inner side of the 
condyle, a second concavity separates the articulation from the small malleus. The 
external lateral aspect of the quadrate is defined from the squamosal by a deep horse¬ 
shoe-shaped groove, which marks posteriorly the limit at which the squamosal over¬ 
laps it (fig. 2). The extreme height of this anterior part of the bone is 5 centims.; its 
extreme antero-posterior extent in front of the squamosal and above its short ptery¬ 
goid process is 2‘5 centims. The squamosal descends in front of it as a slender 
process, which does not reach down to the pterygoid process. The lateral aspect of 
the quadrate bone is divided into two areas by an oblique ridge, which extends 
forward and upward from the anterior termination of the condyle articulation. The 
superior posterior area is higher than wide, flattened, but slightly concave vertically. 
The inferior wedge-shaped area looks obliquely downward, outward, and forward, 'fhe 
pterygoid process is about 1‘2 centim. deep, and 8 millims. long, and about 6 millims. 
thick, compressed towards the inferior and superior margins. It is continuous with 
the quadrate process of the pterygoid, from which it is divided by a vertical sut.ure. 
Posteriorly the quadrate bone passes obliquely through the squamosal so as to occujjV 
a large area on the posterior face of that bone. In this species the quadrate bone has 
a general resemblance to the quadrate of Ichthyosaurus, though that genus does not 
develop a pterygoid process, which I have seen in no other Dicynodont. In Dicyn- 
odon leoniceps the quadrate bone is of different form, and has no extension in front 
of its condyles, but above the trochlear end it rises in a quarter of a circle, vertical 
externally, and convex on the inner border, 5^ centims. high, and as wide as high, 
just above the articulation. It is strong, very slightly convex, and makes an angle of 
nearly 45° with the longitudinal articulation. Its antero-posterior thickness above 
the condyle is 4^ centims., and superiorly the posterior surface, which was contained 
within the squamosal bone, is convex from behind, upward and forward toward the 
sharp superior margin, which, as preserved, projects forward centim. in advance of 
the squamosal bone. Its external margin appears to have been overlapped and 
hidden, as in all the other species, by a descending squamosal process. 
On the palatal aspect (fig. 3) there is a dee23 saddle-shaped channel over the two 
downwardly directed processes, which appear to be formed by the basi-occipital and 
basi-sphenoid bones, which is very convex from behind forward, because the processes 
converge in front. Each process is 2-| centims. long, and has a long ovate form. The 
transverse measurement over them is less than 3 centims. On the outer posterior 
side, wedged in between the basi-occipital part of the process and the quadrate bone, 
is a relatively small ossification, identical with that which I have recognized in other 
species and regard as the malleus. It extends obliquely outward and backward. Its 
surface is less than a centimetre square, and is obliquely convex from behind forward. 
External to the convex sphenoido-occipital processes the posterior quadrate bars of 
the pterygoid, which are 2‘5 centims. long and about 1‘5 deep, converge forward and 
inward. They do not extend further forward than the processes with which they are 
MDCCCLXXXIX.—B. 2 H 
