42 



THE CERATOPSIA. 



mf 



THE SURANGULAR. 



This is the largest of the three bones of this region. Its shape is very irregular and it may 

 best be described as consisting of a larger vertical portion expanded antero-posteriorly and a 

 smaller and pointed horizontal portion. The vertical portion is in contact anteriorly with the 

 dentary and at its superior angle it is rounded and thickened, and when in position it fits into a 

 deep pocket in the external wall of the dentary at the base of the 

 coronoid process. Posteriorly and externally on the superior surface 



it articulates with the quadrate, while inter- 

 nally and superiorly it is overlapped by the 

 articular. Medially on the inner surface 

 there is a long, angular projection, which em- 

 braces the antero-external surface of the 

 articular. Inferiorly and internally the sur- 

 angular is in contact with and overlaps the 

 angular, being wedged in between that bone 

 and the articular. Postero-externally the 

 surangular entirely incloses the mandibular 

 fossa and the external mandibular foramen 

 is reduced to a small hole situated entirely 

 within this bone. Posteriorly the surangular 

 does not extend beyond the articular sur- 

 face for the quadrate, and just beneath the 

 posterior border of this surface the bone is 

 pierced by a foramen of medium size. 



Fig. 38.— A, Superior view of left 

 angular of type of Tricera- 

 tops flabellatus, No. 1821, 

 Yale Museum; B, inferior 

 view of same, a, External 

 border; p.posteriorextrem- 

 ity; ss, surface for surangu- 

 lar; sar, surface for articu- 

 lar; sd, surface for dentary; 

 es, exposed surface; sp, 

 splenial surface. One-eighth 

 natural size. 



THE ARTICULAR. 



Fig. 39.— A, External view of right 

 surangular of type of Tricera- 

 tops flabellatus, No. 1821, Yale 

 Museum; B, internal view of 

 same, a, Supero-anterior angle; 

 sd, surface for dentary; mf, ex- 

 ternal mandibular foramen; asq, 

 articular surface for quadrate; 

 sar, surface for articular;ma, mar- 

 gin for contact with angular; sa, 

 surface for contact with angular. 

 One-eighth natural size. 



The articular is' embraced below by the 

 angular and surangular, while on its external 

 side it is firmly lodged between the superior and inferior horizontal branches of the latter bone ; 

 together with these bones it entirely incloses posteriorly the mandibular fossa. It forms 

 about three-fourths the articular surface for the quadrate and sends backward a triangular 

 process which extends some 50 millimeters beyond the quadrate and is deeply excavated on 

 its superior surface. The antero-internal margin of the articular is produced forward, especially 



along its superior border, where 



M 



it forms a sharp process, trian- 

 gular in cross section, which, 

 when in position, is in contact 

 with the splenial and the pos- 

 terior internal angle of the den- 

 tary. It forms the posterior 

 and a portion of the superior 

 border of the internal mandib- 

 ular foramen. 



The manner in which the 

 angular, surangular, and artic- 

 ular interlock with one another is well shown in fig. 40, which represents all these bones in 

 their normal position relative to one another. 



Although the bones of this region are short and small and are not coossified either with 

 themselves or the dentary, except perhaps in very old individuals, yet when adjusted to each 

 other and the dentary they interlock by such a complicated system of sutures, cavities, and 



Fig. 40.— A, Posterior view of right angular, surangular, and articukvr of type of Triceratops 

 prorsus. No. 1822, Yale Museum; B, oblique front view of same; C, external view of 

 same, ar, Articular; an, angular; sa, surangular; /, foramina; emf, external mandibu- 

 lar foramen. One-eighth natural size. 



