456 
PEOrESSOR OWEN ON THE DICYNODONT EEPTILIA. 
Ptychognathus Alfred% Owen. 
By the angular contour of the profile (figs. 1 & 3), in which the almost straight line 
of the vertex {v o) meets the equally straight occipital line (o h) at a right angle, and 
the straight facial or naso-premaxillary line {v n) meets that of the vertex at an obtuse 
angle, the specimen belongs to the subgeneric form of the dicynodont family called 
Ptychognathus. 
The occiput (fig. 4) shows the same crocodilian extent of ossification as in other 
members of this singular family. 
The articular tubercle is formed, as in Ptychognathus declivis*^ by the basioccipital 
(fig. 4, i) at the lower and middle part, and, in less proportions, by the exoccipitals {ib. 2 ) 
which just meet at the upper part of the tubercle. The foramen magnum is a rather 
narrow oval, with the small end upwards, exclusively bounded, externally, by the exocci- 
pitals ; the basioccipital appearing at the interspace of the exoccipitals immediately 
within the cranial cavity. The basioccipital in advance of the tubercle, has its lower 
surface at first concave, and then convex, lengthwise ; and is more deeply concave trans- 
versely (fig. 5, 1 ) through the production of its sides into the short and thick hypapo- 
physes, figs. 4 & 5, hy. Its junction with the basisphenoid (fig. 5, «} is by an almost 
straight transverse harmonia. The hypapophyses {hy) are divided by a short notch from 
the paroccipitals (fig. 4, 4 ), which abut, as in Crocodilia and Chelonia, against the inner 
side of the tympanies ( 23 ), but here against their lower half, fixing these bones more 
firmly in their position. The paroccipitals, which, as in the Crocodile, are exogenous 
growths of the exoccipitals {ih. 2 ), are divided by a wider and more shallow notch from 
the exoccipital and mastoid, a. The superoccipital (fig. 4, 3 ) forms the upper part of 
the occipital surface, is of a vertically oblong or subtriangular form, with the apex 
downward, and terminating some distance above the foramen magnum. The sides of 
the superoccipital concavity are formed by the backwardly projecting and slightly 
diverging hinder end of the parietals (fig. 4, 7 ) ; between which and the exoccipitals and 
tympanies the broad and angular mastoid [ih. s) are wedged, forming the upper, outer, 
and posterior outstanding backwardly bent ridge, for the attachment of the cervical 
muscles, and presenting the sutural surface to the upper end of the tympanic, 28 . The 
upper surface of the parietal (fig. 2 , 7 ) presents a roughish non-articular tract of about 
8 lines in breadth, at the fore part of which is the venous foramen. This median tract 
is made slightly concave across by the elevation of the ridges (#, t) bounding the smooth, 
longitudinally concave, tracts affording origin to the temporal muscle. 
The frontals (n, n) form the chief part of the flattened vertex or upper platform of the 
cranium (figs. 2 & 4, f, t). They extend longitudinally from the foramen parietale 
forward to about one-third of an inch from the prefronto-nasal ridge (fig. 2, f); and 
transversely to the superorbital ridges (s), of which they form about the middle third 
part. The median or frontal suture may be traced along a great part of their extent : 
the surface is slightly raised at this part, and is depressed laterally, between the median 
* Quarterly J oumal of tlie Geological Society, vol. xvi. (April 1859) p. 49, pi. 1. figs. 3-5. 
