THK SKULL 



31 



would h.u c been attached were they present. This is apparently true also of a Chasmosaurus skull in 

 the Yale collection, but not in either Centrosaurus skull under discussion. Hatcher states that in 

 Triceratofs the supraorbital horns are simple outgrowths of the postfrontals, whereas the nasal horn, 

 like the rostral, predentary, epoccipitals, and epijugals, is to be considered a dermal or epidermal 

 ossification. But no known Triceratofs skull is individually young enough to prove the point, for 

 the supraorbital horns are so large that coalescence with the postfrontals must have occurred at a very 

 early age if they arose from separate centers of ossification. Brown expresses doubt as to whether 

 the supraorbital cores bore a horny sheath in the rudimentary stage of their development in Centro- 

 saurus. But judging from the abundance of vascular impressions, especially on the crest of older 



Y. P.M. 2015 



Fie. 4. — Left lateral view of skull of Monoclonius (C ' entrosaurus) flc.xtts, at Yale Peabody Museum, T s natural size. 

 an, angular; ap, anterior process of crest; ar, articular; c, coronoid process; d, dentary; e, epoccipital; »/, epijugal; 

 /, prefrontal; h, supraorbital horn; /'/, preorbital fossa; «'//, infratemporal fossa; /', jugal; /, Iacrymal; mx, maxillary; 

 n, nasal; nar, narial opening; nk, nasal horn core; 0, orbit; p, parietal; pd, predentary; />/, parietal fenestra; pmx, pre- 

 maxillary; po, postfrontal; pp, posterior process of crest; q, quadrate; y;', quadratojugal ; r, rostral; .<,;, surangul.ir; 

 sj, squamosal; /, teeth. 



individuals, the entire head except for muscle areas must have been clothed with a close-fitting re 

 tant integument, leathery if not actually horny in texture. It is reasonable to assume that the areas 

 of the horns were likewise so covered, and that, as in the horned lizard P/irynnsoDia, corneous horn- 

 like prominences may actually have preceded rather than followed the development of an osseous 

 horn core. 



Rostral 



This bone is essentially as in Triceratofs except for its smaller size relative to the entire face. 

 The ascending process does not rise so high on the nose in Centrosaurus, and as a consequence the 

 posterior margin of the rostral is straighter. The highly rugose and pitted character of the bone is 

 characteristic of all ceratopsians. The muzzle of Centrosaurus is relatively deeper than in Triceratofs 

 and much deeper and shorter than in the contemporary Chasmosaurus. 



