oabsh.] TRICERATOPS. 209 



horn core. Hence these parts are represented in outline, taken from 

 another specimen in which they are all present and in good preservation. 

 In another skull represented (PI. LIX, and PI. LXI, figs. 1-3), the 

 rostral bone and nasal horn core are in position and firmly coossified 

 with the adjoining elements. 



The premaxillary bones are large and much compressed transversely. 

 Their inner surfaces are flat and meet each other closely on the median 

 line. In old specimens they are firmly coossified with each other and 

 with the rostral bone. Each sends upward a strong process to sup- 

 port the massive nasals. Another process, long and slender, extends 

 upward and backward, forming a suture with the maxillary behind, and 

 uniting in front with a descending branch of the nasal. The premaxil- 

 laries are much excavated externally for the narial aperture, and form 

 its lower margin. They are entirely edentulous. 



The maxillaries are thick, massive bones of moderate size, and sub- 

 triangular in outline when seen from the side. Their front margin is 

 bounded mainly by the premaxillaries. They meet the prefrontal and 

 lachrymal above, and also the jugal. . The alveolar border is narrow 

 and the teeth are small, with only a single row in use at the same time. 



The nasal bones are large and massive, and greatly thickened ante- 

 riorly to support the nasal horn core. In two of the skulls figured 

 these bones are separate, but in older individuals they are firmly coossi- 

 fied with each other and with the frontals. The nasal horn core ossifies 

 from a separate center, but in adult animals it unites closely with the 

 nasals, all traces of the connection being lost. It varies much in form 

 in different species. 



THE HORX CORES. 



The frontal bones are quite short and early unite with each other and 

 with the adjoining elements, especially those behind them. The frontal 

 or central region of the skull is thus greatly strengthened to support the 

 enormous horn cores which tower above. These elevations rest mainly 

 on the postfrontal bones, but the supraorbitals and the postorbitals are 

 also absorbed to form a solid foundation for the horn cores. 



These horn cores are hollow at the base (PL LX, fig. 3), and in gen- 

 eral form, position, and external texture agree with the corresponding 

 parts of the Bovidfe. They vary much in shape and size in different 

 species. They were evidently covered with massive, pointed horns, 

 forming most powerful and effective weapons. 



The orbit is at the base of the horn core, and is surrounded, espe- 

 cially above, by a very thick margin. It is oval in outline and of mod- 

 erate size. 



The postfrontal bones are very large, and meet each other on the 

 median line. Posteriorly they join the squamosals and the parietals. 

 At their union with the latter there is a median foramen (PI. LX, fig. 

 3, x), which may correspond to the so-called "parietal foramen." In 

 old individuals it is nearly or quite closed. When open it leads into 

 10 GOGOL, pt 1 14 



