OSTEOLOGY OF CARNIVOROUS DINOSAURS. 



89 



On the internal side the dentary is overlain by the splenial and supradentary. 



Posteriorly the dentary sends back a thin pointed process on the lower border, 

 which laps along the outer side of the angular, ending anterior about the middle 

 of the external mandibular foramen. 



The complete coalescence of the dentary and surangular renders the point 

 of their union undetermined, though on the external surface above the external 

 mandibular foramen there is indication of a second lapping process of the dentary 

 upon the surangular, but of this one can not be sure. 



On the external surface of the dentary are the usual foramina, which doubt- 

 less served for the transmission of nerves and nutrient blood vessels to the lips. 



Supradentary 1 (d.p.). — The supradentary in Ceratosaurus, recognized in this 

 genus for the first time, is a long, thin bone, that extends along the upper, inner 

 side of the dentary (fig. 55). It extends nearly the entire length of the dental series 

 and hides from an internal view the greater portion of the dental "rugosae." The 

 upper border of the anterior half of this bone is slightly below the tops of the 

 rugosae, but on the posterior half it rises above them to form the inner parapet of 



Fig. 55.— Internal raw or leftramus of Ceratosaurus nasicornts Marsh. No. 4735, TJ.S.N.M. \ nat. size, an, angular; 

 ar, articular; d, denatry; d.p, supradentary; ex.m.f, external mandibular foramen; par, prearticular; sp, 

 splenial; sur, surangular; 10, root of tenth tooth; IS, indicates position of last alveolus. 



the alveoli, and in that way it contributes somewhat to the support of the teeth. 

 The anterior end is missing, but apparently it was pointed and ended just posterior 

 to the most anterior tooth, as shown in figure 55. 



Hay 2 regarded this element as being the anterior portion of the splenial, but in 

 this he certainly was mistaken, for on examination of the specimen the splenial 

 is plainly seen to overlie this element. 



In Tyrannosaurus Osborn 3 concludes that the supradentary is "apparently 

 of splenial origin, but suturally separated from it." I fail to find any evidence 

 to bear out this assertion in the material now at hand. 



Angular (an.). — In the first restoration of the Ceratosaurus skull by Marsh 

 (pi. 8, fig. 1, Dinosaurs of North America), no attempt was made to differentiate 

 the elements of the posterior half of 'the jaw. Hay was the first to recognize these 

 bones, though his description of them is very brief. The angular, somewhat dis- 

 placed, is plainly shown on the right ramus (fig. 1, pi. 17). It is a thin, flattened ele- 

 ment that articulates by squamous union with the lower median portion of the 



1 Name proposed by Osborn in 1912 (Mem. Amer. Nat. Hist., vol. 1, p. 5). Lambe in 1904 recognized this same element in 

 Albertosaurus but failed to give a distinctive name, designating it as the "inner alveolar plate of dentary." (Contrib. to Canadian 

 Paleontology, vol. 3, pt. 3, 1904, pt. 11.) 



« Proc. U. S. Nat. Mus., vol. 35, 1908, fig. 4. 



3 Mem. Amer. Mus. Nat. Hist., vol. 1, pt. 1, 1912, p. 24. 



