258 PALEONTOLOGY. 



been subgenerically separated under the name Ptychognathus. 

 Their remains characterize the same formations as those of 

 Dicynodon. 



Ptychognathus* declivis, Ow. — In the skull of this species 

 (fig. 96, 3a) assuming the horizontality of the upper (fronto- 

 parietal) plane of the cranium as giving the natural position 

 of the skull, the broad plane of the occiput meets it at an 

 acute angle, rising from the condyle upward and backward — 

 a direction not previously observed in any reptile, and similar 

 to that presented by the occiput in relation to the vertex in 

 many mammals. 



The fronto-parietal plane (ib. 3 b) is bounded by an anterior 

 ridge (3 b, 14, 15), whence the facial part of the skull (3a, i& ») 

 descends in a direction nearly parallel with that of the occi- 

 put. The occipital ridge (3 c, 7. 8) is notched at the middle. 

 The occipital plane, owing to the outward expansion of the 

 masto-tympanic plates (3 c, 8, 28), becomes the broadest part of 

 the skull, which quickly contracts forward to the ridged 

 beginnings of the alveoli of the canine tusks (3 b, 21). 



The nostrils (3 a, n) are situated nearer the orbits (0) than 

 the muzzle. They are proportionally smaller than in the 

 typical Dicynodonts. The orbits (0) are so placed and shaped 

 as to suggest that the reptile had the power of turning the 

 eye-ball so as to look upward and backward, as well as out- 

 ward, in a peculiar degree. The upper outlets of the temporal 

 fossae are broader than they are long. The palate has a single 

 large oval vacuity at its back part, bounded externally and 

 behind by palato-pterygoid ridges. In one orbit, a few scle- 

 rotic plates (3a, s.) were preserved. 



The occipital condyle (3 c) is subtrilobate, and is formed 

 by the basi-occipitals (ib. 1) and ex-occipitals (ib. z ) in equal 

 proportions : the latter have coalesced, as in the crocodiles, 

 with the paroccipitals (ib. 4 ). The parietals form one bone, 



* From ptyx, a fold or ridge, and gnathos, a jaw. 



