THE SKULL 33 



and maxillary occurs. In Centrosaurus as we have seen, the nasal forms part of the septum separat- 

 ing the anterior nares. The extent of this septum area varies in the different Cenlrosaurus skulls. 



In Triceratops, the nasals have no part in the nasal septum. /'C^ u 



Lacrymal 



* 

 The lacrymal is, comparatively, of about the same size as in Triceratops, but differs from the^. v 



latter in being sometimes bounded anteriorly by the posterior process of the premaxillary. Below, iM^ - 



is bounded by the jugal and maxillary, and above, by the nasal and prefrontal. 



Maxillary 



This element has the same form and limitations as in Triceratops. It contains alveolar grooves 

 for about 31 teeth, and on its inner face has a corresponding number of nutritive foramina in a hori- 

 zontal row. The preorbital fossa is included within the maxillary though generally at the upper 

 border. The maxillary is bounded in front and above by the premaxillary, above by the lacrymal 

 and jugal, behind by the jugal, and on the roof of the mouth by the premaxillary, palatine, and 

 pterygoid. 



Jugal 



The form of this bone differs markedly from that in Triceratops, being broader and more 

 rectangular, whereas in the latter the outer aspect of the bone is somewhat T-shaped. The area 

 which overlaps the quadratojugal is much broader in Centrosaurus. Externally, the jugal is bounded 

 by the maxillary and lacrymal, in front and above, and posteriorly by the postfrontal (postorbital) 

 and squamosal. The epi jugal is borne on the lower outer corner of the jugal as in all ceratopsians. 

 Posteriorly, the infratemporal fossa is entirely surrounded by the jugal and squamosal, the quadrato- 

 jugal not forming the lower border as it does in Triceratops. The jugal of Centrosaurus covers 

 over more of the quadratojugal-quadrate complex than in Triceratops, when viewed squarely from 

 without. 



Squamosal 



This is a most characteristic bone in the Ceratopsia, concerning which there has been little or no 

 discussion, as all authorities agree as to its identity. Its form and development are of generic value. 

 The squamosal of Centrosaurus is roughly quadrangular with the Jong axis running obliquely down- 

 ward and backward. Its upper forward margin articulates with the postfrontal. Anteriorly, it 

 meets the jugal, posteriorly the parietal. The outer free margin sends forward a short process 

 which forms part of the lower border of the infratemporal fossa and unites with the jugal and 

 quadrato-jugal. The anterior free border of the squamosal, together with the quadrate complex, 

 forms a deep notch behind which the border of the squamosal is continued to form the lateral margin 

 of the crest. This part of the border has five prominences which orginally bore the epoccipital bones, 

 of which two only are preserved on the right side and none on the left, where the margin is broken. 

 In this specimen the remaining epoccipitals are fully coossified with the squamosal. In the type 

 specimen of C. fexus there are three on the right side, one of which is represented by the base only. 

 These squamosal epoccipitals in the type are much smaller than are those borne on the parietals, 

 whereas in the Yale specimen they are nearly equal in size. The number of prominences is one 

 greater in the type and the emarginations between them are not so deep. I find five to be the usual 

 number of squamosal epoccipitals, not only in Centrosaurus but in Styracosaurus also. In Anchicera- 

 tops the number is six, while in Triceratops it is generally six with the seventh overlapping the 

 squamoso-parietal suture. In long-squamosal forms, like Chasniosaurus, the number is naturally 

 greater, and averages nine. 



