30 



THE CEBATOPSIA. 



posterior extremity is bifurcated to embrace the compressed median portion of the alisphe- 

 noids, as shown in fig. 27, and the lateral processes of the vomer extend a little back of the 

 posterior border of the palatines, by which they are in turn embraced. The inferior border 

 of the bifurcated posterior extremity of the vomer is thickened above the pterygoids, and 

 each process presents on its inferior surface a groove into which fits the short, wedge-shaped 

 superior border of the apex of the pterygoid, and the suture thus formed between the vomer 

 and the pterygoids is W-shaped, as shown in fig. 25. Above their contact with the pterygoids 

 the vomers are firmly inclosed throughout their breadth by the palatines. 



THE PREMAXILLAEIES. 



The premaxillaries are 

 edentulous. They are much 

 compressed and closety applied 

 to each other. The inferior 

 border is heavy and posteriorly 

 it sends upward and backward 

 a strong process, which is 

 wedged in between the maxil- 

 lary and the posterior descend- 

 ing branch of the nasal, send- 

 ing upward a long, slender 

 wedge which is inserted into 

 a deep groove in the latter. 

 The supero-anterior border of 

 the premaxillary describes the 

 arc of a circle. It is thin below, 

 but thickened above where it 

 meets the anterior extremity 

 of the nasals at the base of the 

 nasal horn. At the summit 

 it is embraced externally by 

 a short but stout descending 

 process of the nasal The nasal 

 is supported from beneath by 

 a strong buttress that runs 

 downward and backward. The 

 external border of this bar or 

 buttress is thick and curves 

 forward. It overhangs a broad 

 triangular fossa of which it 

 forms the posterior border. 

 This fossa is confluent below with that of the opposite premaxillary, though they are separated 

 above by a broad, thin median septum. Posteriorly it connects with the narial orifice by a long 

 foramen. Just back of the base of the buttress which gives support to the superior border of the 

 premaxillary a similar buttress runs obliquely backward and upward, supporting laterally the 

 median septum, which in this region forms the imperfect partition between the nasal openings of 

 opposite sides. The posterior opening of the foramen which connects with the triangular fossa 

 mentioned above lies just beneath and at the base of this buttress. Two foramina (sometimes 

 three), situated one behind the other, may be seen on the inferior surface of the premaxillary 

 near the anterior extremity and well within the external border. Anteriorly the premaxil- 

 laries are embraced and held rigidly in position by the powerful rostral bone, which extends 

 well backward both superiorly and inferiorly. 



Fig. 27.— Vertical section through skull of Triceratops horridus Marsh (type), No. 1820, Yale 

 Museum. Drawn from a fracture just in front of orbits, k, Supraorbital horn core; 

 pf, postfrontal; /, frontal; o, orbit; al, alisphenoid; 61, exit for olfactory nerve; v, 

 vomer; pi, palatine; pt. pterygoid; rax, maxillary; dg, dental groove; /, vacuity between 

 palatines and alisphenoids. One-eighth natural size. 



