NO. 1660. SKULL OF TRICERATOPS—HAY. 101 



We must noAv investigate further that opening on the inner surface 

 of the brain-case (Phite 2, fig. 1, 8^ 10) from which diverge tlie 

 two short canals considered above, the one to the fenestra ovalis, the 

 other to the foramen lacerum posterius. On comparison with the 

 alligator there can be no doubt that we have here a confluence of the 

 opening for the auditory nerve and that for the transmission of the 

 ninth, tenth, and eleventh nerves, the lower part of the foramen hav- 

 ing been devoted to the auditory, the upper part to the other nerves. 

 In the alligator the latter nerves pass out through a long fissure, 

 which is separated from the foramen for the auditory nerve by only 

 a narrow process of bone. Had the cartilage of this process not be- 

 come ossified the two openings would have appeared in the dried 

 skull as a single one, as it does in Triceratops. 



Hatcher thought that the foramen for the facial nerve, his internal 

 auditory meatus, communicated with the small cavities wdiich he 

 represented in his fig. 31 and indicated by the numeral III. The 

 cavities shown there are quite certainly sections of the anterior and 

 posterior semicircular canals, the larger section being the commissure 

 of the two canals. The anterior section belongs in the prootic bone, 

 the posterior in the opisthotic portion of the exoccipital, while the 

 larger section is in the line of union of the two bones. The supraoc- 

 cipital bone must have descended nearly to the level of these sections. 

 Communication w4th these canals was had from Avithin the skull by 

 means of the foramen transmitting the eighth nerve, from without by 

 means of the fenestra ovalis. In all these respects we have here the 

 normal reptilian condition, and we have no reason for thinking that 

 the Ceratopsia were deficient in hearing. 



Now, the fenestra ovalis lies between the prootic bone and that part 

 of the exoccipital that had its origin from the opisthotic. The suture 

 between the prootic and the exoccipital may then be drawn through 

 the fenestra ovalis and carried upward to the supraoccipital, as it 

 runs in the alligator and in Gamptosa\irus. 



Slightly behind and about 28 mm. below the foramen ovale, that 

 exit for the fifth nerve, is the mouth of the canal for the internal 

 carotid artery (Plate 1, fig. 1, car). The other end of the canal is 

 found in the pituitarj^ fossa (Plate 2, fig. 1, car). The hinder open- 

 ing appears to be in the posterior end of the basisphenoid bone. A 

 long shallow groove (Plate 2, fig. 1, car. g) on the underside of the 

 basioccipital process leads forward to it. From the front end of the 

 pituitary fossa a short canal (Plate 2, fig. 1, op. j ; Plate 1, fig. 1. 

 op. f) runs forw'ard and opens on the outer surface of the bone. The 

 outer opening is indicated in Hatcher's fig. 8 by the letters pf. It 

 seems probable that this canal conveyed to the orbit the ophthalmic 

 branch of the internal carotid artery, a vessel that in man escapes 

 through the optic foramen. 



