THE SKULL. 



41 



fossa. Inferiorly and laterally this fossa is partially inclosed by the splenial, and posteriorly 

 by the angular, surangular, and articular. On the superior border and near the anterior end 

 of this cavity two large foramina pierce the dentary and probably conveyed blood vessels to 

 the interior of that bone. On the inner side of the dentary, about midway between the superior 

 and inferior borders, there is a series of foramina that extends throughout the entire leno-th 

 of the bone. These foramina equal in number the teeth of the vertical series, and doubtless 

 served for the transmission 

 of nerves and nutrient blood 

 vessels to the teeth. a Pos- 

 teriorly the dentary was in 

 contact with the angular, sur- 

 angular, and articular, though 

 never becoming coossified 

 with them, even in old indi- 

 viduals. 



The more important 

 characters of the dentary are 

 well shown in fig. 36, drawn 

 from the right dentary of the 

 type of Triceratops flabettatus, 

 No. 1821, Yale Museum. 



THE SPLENIALS. 



of ™f 



These are very long, thin 

 bones, especially slender an- 

 teriorly. They reach to the 

 symphyseal border in front, and posteriorly they inclose internally and inferiorly the mandibu- 

 lar fossae. Throughout their entire length they are closely applied to the dentary, though they 

 seldom, even in the old individuals, become coossified with it. In the middle and posterior 

 region of the splenial the superior margin is expanded and overlaps the internal surface of the 

 dentary above the lateral border of the mandibular fossa. Posteriori}^ a little beneath the 

 superior margin, there is a long slit dividing the posterior portion of the splenial into a broad, 

 long inferior branch and a shorter and narrower superior. This slit or fissure is homologous 

 with the internal mandibular foramen in the crocodile. It is inclosed behind by the articular. 



The inferior and larger of the two posterior 

 branches of the splenial overlaps the angular 

 throughout the entire length of the latter bone. 



THE ANGULAR. 



This is a broad, thin bone, convex on its 

 inferior or external surface and concave above 

 where posteriorly it receives the articular and sur- 

 angular, while its thinner and narrower anterior 

 portion overlaps the dentary, passing within the mandibular fossa and forming a portion of 

 its postero-inferior wall. The external border of the angular is the thicker and is in contact 

 throughout with the surangular, while medially it is produced into a rounded angle which fits 

 into a corresponding pocket or groove in the surangular. On its inferior surface it is pierced 

 by a small foramen which leads into the mandibular fossa. It is the smallest of all the elements 

 of the mandible and its form and characters are well shown in fig. 38. 



Fig. 36.— A, external view of right dentary of type of Triceratops fiabellatus, No. 1821, Yale 

 Museum; B, internal view of same, c, Coronoid process; af, dental foramina; sp, surface 

 for predentary; ss, symphyseal surface; mf, mandibular fossa. One-eighth natural size. 



Fig. 37. — Internal view of left splenial of type of Triceratops 

 prorsus, No. 1822, Yale Museum, a, Anterior end; p, pos- 

 terior; imf, internal mandibular foramen. One-eighth natural 

 size. 



a See footnote on p. 20.— R. S. L. 



