40 



THE CEKATOPSIA. 



the rostral sheath fitted over that of the predentary, exhibiting relations similar to those which 

 obtained between the superior and inferior dentitions where, as will be shown later, the external 

 margins of the inferior teeth oppose the internal margin of the superior, the lower jaw fitting 

 within the superior as in mammals. 



On the superior surface of each lateral branch of the predentary there is a broad, shallow 

 groove for the reception of the postero-inferior branches of the rostral and the antero-inferior 

 border of the premaxillary. The form and principal characters of the predentary are well 

 shown in fig. 35. 



Fig. 35. — A, Side view of predentary oi type of Triceratops prorsus, No. 1822, Yale Museum; B, superior view of same; C, inferior view; D, 

 posterior view; E, anterior view, a, Anterior end; p, posterior end; i, inferior process; s, superior process. One-eighth natural size. 

 After Marsh. 



THE DENTAEY. 



This is much the largest of the bones of the lower jaw. Anteriorly it is much compressed, 

 and it was united with the opposite dentary by cartilage only, through a short, grooved sym- 

 physis. A little posterior to the symphyseal surface the transverse diameter of the dentary 

 increases rapidly, and throughout its entire extent it is a strong, heavy bone. Posteriorly 

 on its external surface it sends upward a heavy coronoid process, compressed transversely 

 but much expanded antero-posteriorly at the summit and somewhat resembling the same 

 process in some mammals. The coronoid process is constricted antero-posteriorly below the 

 summit, but thickened transversely. It is exceptionally strong, and the surface of the extremity 

 is rugose and doubtless served for the attachment of the powerful muscles which in life aided 

 in closing the lower jaw. Immediately beneath the base of the coronoid process there is a deep 

 cavity opening on the inferior surface of the dentary throughout about one-half its length. 

 This cavity, which is present in most reptilian rami, is very appropriately called the mandibular 



