52 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Apr. 20, 



The zygomatic arch is a deep compressed plate of bone, with a 

 convex upper border inclining a little outward, formed chiefly by the 

 mastoids, which join the squamosal and malar near the lower and 

 back part of the orbit, — the squamosal there being wedged between 

 the mastoid and malar, forming the infero- anterior boundary of the 

 temporal fossa. Between the zygomatic and tympanic plates a wide 

 and deep oblique channel is included, which expands as it extends 

 obliquely downward and forward. 



The prefrontals, as in Pt. declivis, develope the superorbital tu- 

 berosities and the outer part of the ridge dividing the upper from 

 the fore part of the cranium. The postfrontals form the hinder 

 boundary of the orbits. The frontals contribute a small part to 

 the upper boundary. The lacrymal forms the infero-anterior boun- 

 dary, and extends a short way upon the face. A bone below the 

 nostril appears in the present skull to be marked out by a fissure on 

 each side from the maxillary ; it may be a dismemberment of the 

 lacrymal. The nasals are a pair of broad bones, each of a rhomboidal 

 shape ; they form the middle part of the anterior cranial ridge, behind 

 which they unite with the frontals and prefrontals : their lower and 

 front borders diverge to receive the upper part of the long pre- 

 maxillary, and to form the upper boundary of the nostrils. Each 

 nasal, in advance of the precranial ridge, presents two facets, the 

 outer one bending down to join the facial part of the prefrontal. 



The superior maxillary presents a deep facial plate, proportionally 

 deeper behind than in Pt. declivis : its postero-inferior part is pro- 

 duced into a slender pointed process, underlapping and on the inner 

 side of the malar, below the orbit: in advance of this, the bone 

 rapidly expands, a ridge dividing the outer from the under part of 

 the bone : this is not continued directly into the alveolar ridge, the 

 latter beginning to rise a little above the termination of the former 

 ridge. Below this termination and the beginning of the alveolar ridge, 

 the maxillary sends down an inequilateral triangular plate to join the 

 palato-pterygoid boundary of the palatal nostril. Above this plate 

 the maxillary expands to form the socket of the canine tusk, which 

 is strengthened by the strong ridge on its outer part. The upper 

 part of the maxillary forms the lower half, or more, of the side of the 

 face, and terminates anteriorly above the alveolus, by forming a small 

 part of the alveolar border. This border, anterior to the tusk, is 

 continued obliquely upward and forward to the middle of the pre- 

 maxillary bone — a peculiarity of contour which demands a corre- 

 sponding production of the fore part of the mandible to close the 

 mouth. 



The premaxillary is a long single bone ; if it were ever divided at the 

 middle line, the suture has been obliterated : the bone has all the 

 appearance of having been single, as in Birds and most Lizards. It 

 is of unusual length : beginning above by the pointed termination 

 wedged between the nasals, it expands to the fore part of the nostrils, 

 the sides of the bone there beginning to bend down at an open angle 

 • (nearly approaching a right one) with the upper surface ; this surface 

 maintains almost the same breadth to the alveolar border: it is 



