OSTEOLOGY OF CARNIVOROUS DINOSAURS. 



19 



Viewed from the side this end is ovally emarginated, thus forming the nasal contri- 

 bution to the boundaries of the external narial opening. 



On the median ventral side the bone is traversed longitudinally by a shallow 

 groove that marks the union of the nasal with the underlying maxillary. At the 

 center and external to this groove the nasal overhangs the external surface of the 

 maxillary, instead of rounding evenly into it as in most dinosaurian skulls. On the 

 underside of this overhanging portion is one large opening, flanked by two smaller 

 ones, that lead into connecting chambers within the heavier part of the bone. As 

 observed in several of the larger elements of the skull, these chambers are evidently 

 a provision of nature for getting the maximum strength from a'minimum weight of 

 bone, a feature that goes well with the light and open structure of the skull. 



TIlX. 



ynx 



Fig. 10.— Right premaxillary of Antrodemus valens Leidy. No. 4734, U.S.N.M. \ nat. size. (A) lateral view; (B) 



INTERNAL VIEW; a, ANTERIOR NARIAL OPENING; mi, END JOINING MAXILLARY; na, EXTENT OF UNION WITH THE NASALS. 



The sharp superior external edge, with the roughened central external margin 

 of the nasal, is suggestive of having had a chitinous covering. 



When articulated the median superior surfaces of the nasal region is decidedly 

 dished transversely. 



At the center the nasal has a transverse diameter of 56 mm. ; near the posterior 

 end, 45 mm. ; near the anterior end, 15 mm. 



Premaxillary (pmx.). — The premaxillary is a heavy rectangular bone, with a 

 long, slender, tapering process that projects upward and backward from the superior 

 border anteriorly and forms part of the superior boundary of the external nasal open- 

 ing. It unites above the nares with the nasals by a long lapping sutural contact. 

 The anterior end presents a flattened surface, oblique to the longer axis of the bone, 

 for union with the premaxillary of the opposite side (fig. 10). On the internal side 



