THE SKULL. 



19 



anterior opening of the alisphenoid canal there is a rather deep pocket, at the base of which 

 are four foramina. The two larger of these foramina occupy a more elevated position than the 

 two smaller, and are placed one anterior to the other. The anterior and slightly larger of 

 the two is the optic foramen, while the posterior is doubtless homologous with the sphenoidal 

 fissure. The two smaller foramina (pf, fig. 8) lead directly into the comparatively deep and 

 elongate pituitary cavity, and their homologies are not definitely known. They may have 

 served to transmit nerves to the large muscles of the orbit. The positions of the various 

 foramina mentioned above are well shown in- fig. 8, which represents the right side of the 

 external wall of the brain case with the supraoccipital and postfrontal in position, as seen 

 obliquely from beneath and in front. In so far as I have been able to determine, the position 

 of these foramina is fairly constant in the various genera and species of the Ceratopsida?. 



THE FRILL OR PARIETAL CREST. 



The parietals and squamosals are enormously expanded and form a great hood-like frill, 

 which projects far beyond the occipital segment of the skull, entirely inclosing both superiorly 

 and laterally the cervical region. In life the frill doubtless 

 afforded considerable protection to the neck, especially in 

 the genus Triceratops, though it could not have been so 

 effective as a protective armor in Torosaurus. The special- 

 ization which has taken place in the bones of the frill has 

 not been uniform in the different genera and species of the 

 Ceratopsidse, so that this region of the skull is of the greatest 

 importance in determining genera and species. In Tricera- 

 tops it consisted of a continuous sheet of bone, composed of 

 the closely applied squamosals and parietals. The inferior 

 surface was deeply concave transversely, and the whole 

 projected backward and extended for a distance of nearly 

 or quite a meter behind the occipital condyle, roofing over 

 the entire cervical region above and extending well down 

 on either side. To increase its effectiveness as a protective 

 shield, and perhaps also at the same time to give to it a cer- 

 tain value as an offensive weapon, its external margin or 

 periphery was curved outward and supported a series of 

 elongated, acuminate, triangular ossicles (epoccipitals), 

 which have the appearance of having been covered in life 

 with some hard chitinous substance. When in position, 

 these give to the border of the frill a scalloped or serrated 

 appearance. In the genus Triceratops these ossicles are derived from separate centers of 

 ossification and do not ankylose with the squamosals and parietals until late in life. In the 

 type of T Jiabellatus, which pertained to a young individual, they were for the most part found 

 in position, but free; while in- the type of T prorsus, representing an old animal, they are 

 firmly coossified with the squamosals and parietals. They are well shown in figs. 10 and 11, 

 as is also the general form of the frill. In the genera Monoclonius and Torosaurus there are 

 no epoccipitals, but the margins of the squamosals and parietals each present a series of 

 more or less distinct prominences, giving a serrated appearance to the margin of the frill, 

 similar to that seen in Triceratops. In the first-mentioned genera, however, these promi- 

 nences are not derived from separate centers of ossification, as are the epoccipitals in Tricer- 

 atops, but are present even in young individuals, firmh T attached to and forming a part of the 

 squamosals and parietals, as shown in figs. 12 and 13 and in Pis. II and III. The variation in 

 the form and structure of the frill in the various genera and species of the Ceratopsidse is well 

 shown in PI. II, where a comparative view of the different types of frill is given. 



Fig. 11. — Posterior view of skull of Triceratops 

 jiabellatus (type), No. 1821, Yale Museum. 

 h, Supraorbital horn core; e, epoccipital; 

 p, parietal; s, squamosal; q, quadrate; pd, pre- 

 dentary; d, dentary. One-twelfth natural 

 size. After Marsh. 



