THE SKULL. 



39 



protruding eyes. These characters, together with the ample dimensions of the optic foramina, 

 suggest that these animals were gifted with keen sight. If we are to judge hy the comparative 

 development of the various sense organs, the Ceratopsia would seem to be extremely deficient 

 in the sense of hearing. Properly speaking, there are no auditory capsules in the skull of the 

 Ceratopsia and the otic bones, if present at all, are so completely fused with those forming the 

 cerebral walls that they are entirely unrecognizable. There is a noticeable swelling on either 

 side in the walls of the brain cavity at the point where the auditory capsule should, if present, 

 be located, as shown in fig. 31. But the walls, instead of being thin at this point, are thick, and 

 instead of a capacious auditory bulla on either side there is only a small reniform cavity termi- 

 nating below in a single median shallow rounded pit and two smaller ones placed one anterior 

 and the other posterior to the median. So far as I can determine two small foramina enter 

 this cavity — one, the smaller, by way of the median round pit mentioned above, the other, the 

 larger, by the anterior of the two 

 smaller of these pits. I am un- 

 able to determine the course or 

 the homologies of these two 

 foramina. The former seems, 

 however, to be a rudimentary 

 auditory foramen, while the 

 larger appears to_ have commu- 

 nicated with the foramen ovale. 

 However this may be, and what- 

 ever the homologies of these and 

 various other foramina, it is 

 evident from the structure of 

 the skull in this region that the 

 sense of hearing, unlike the sense 

 of sight and smell, was exceed- 

 ingly poorly developed in the Ceratopsia. Perhaps this deficiency in hearing may have 

 hastened to some extent at least the extinction of the group. 



Fig. 34.— Lateral view of brain cast of Triceratops scrratm, from No. 2065, U. S. National 

 Museum, c. Cerebral hemispheres; cb. cerebellum; m, medulla; ol, olfactory lobe; on, 

 optic nerve; p, pituitary body; v, fifth nerve, or foramen ovale; x, xr, eleventh and 

 twelfth nerves, or foramen lacerum posterius; xn, twelfth nerve. One-half natural 

 size. After Marsh. 



THE LOWER JAW. 



The mandible in the Ceratopsia is composed of eleven bones. Five of these, the dentary, 

 splenial, angular, surangular, and articular, are paired, while the eleventh, or predentary, is 

 single and median in position, articulating with both rami. 



THE PKEDENTARY. 



The form and principal characters of the predentary are well shown in fig. 35, after Marsh. 

 Seen from below or above it is triangular in outline, deeply excavated superiorly, and with the 

 inferior surface regularly convex. Anteriorly it is compressed, terminating in a median apex, 

 while posteriorly it terminates in three projections, two superior and lateral, each of which 

 overlaps the superior margin of its respective ramus, and one inferior and median, which presents 

 on either side deep grooves into which fit the antero-inferior angles of the rami. The inferior 

 borders of these grooves are projected backward on either side so as to present two posterior 

 branches which overlap the rami and give greater rigidity to the union between the predentary 

 and dentaries. The predentary is wider posteriorly than the rostral but does not extend so far 

 forward as that element, and when in position it is overlapped by the latter, fitting into it as 

 does the lower jaw of a turtle or bird into the upper jaw. It is probable also that the horny 

 coverings with which in life these bones were doubtless enveloped were nicely adjusted to one 

 another and formed efficient cutting organs, serving either as weapons or for procuring food. 

 From the configuration of the rostral and predentary bones it would appear that when closed 



