OSTEOLOGY OF THE ARMORED DINOSAURIA. 43 



pituitary fossa (fig. 10, c). This opening is for the passage of the internal carotid 

 artery. The pituitary fossa is deep, extending well below the brain-case floor, and 

 is wholly within the basisphenoid bone, as shown in plate 10, fig. 3, pi. 



The foramina anterior to the foramen ovale are only shown in specimen No. 

 4936 in these collections (pi. 10, fig. 2). The foramina, located near the central pos- 

 terior border of the orbitosphenoids, represents the exit for the optic nerves (II, fig. 

 2, pL 10). This foramen appears to be divided (not shown in the drawing) by a 

 thin filament of bone, which in an uncrushed specimen would be on the median 

 line. As in Diplodocus, these foramina appear to lie wholly within the orbito- 

 sphenoid bones. 



On the posterior margins of the orbitosphenoids at about their middle, where 

 they coossify with the alisphenoids, there are notches (III, fig. 2, pi. 10), which in 

 all probability give exit to the oculomotor nerves. 



The olfactory foramen is a V-shaped opening formed by the anterior borders 

 of the coossified orbitosphenoids and bounded above by the frontals, and gave exit 

 to the olfactory nerves. This is an unusually large opening (pi. 10, figs. 2 and 3 ol). 



Brain. 

 Plate 10. 



A cast of the cranial cavity of Stegosaurus stenops shows the small size of the 

 brain of this reptile (fig. 5, pi. 10). Its most striking features were the large size of 

 the optic lobes and the small cerebral hemispheres, the latter having a transverse 

 diameter only slightly in excess of the medulla oblongata. The cerebellum was 

 quite small. The peculiar development of the pituitary body is also noteworthy 

 (pi., fig. 5, pi. 10). 



In comparing the brain casts of Stegosaurus and Alligator, Marsh makes the 

 following interesting comments : J 



The contrast in the development of the cerebral region is marked, but in some other respects the 

 correspondence is noteworthy. 



In comparing the proportionate size of the brain of this living reptile with that of Stegosaurus, as 

 given on the same plate, the result proves of special interest. The absolute size of the two brain casts 

 is approximately as 1 to 10, while the bulk of the entire bodies, estimated from corresponding portions 

 of each skeleton was as 1 to 1,000. It follows that the brain of Stegosaurus was only 1 to 100 that of 

 the alligator if the weight of the entire animal is brought into the comparison. 



Marsh concludes that Stegosaurus had, relatively, one of the smallest brains of 

 any known land vertebrate. 



More recently Prof. R. S. Lull makes the following interesting observations on 

 the brain and other parts of the central nervous system. 2 



The brain is remarkable for its extremely small size, the entire cranial cavity, with a length of 

 1.05 cm. and a width of 0.30 cm., displacing but 56 cc. of water and having an estimated total weight 

 of but 2£ ounces. 



The total weight of the animal must have exceeded that of the greatest of living elephants, the 

 brain of which averages 8 pounds, or over 50 times the weight of that of Stegosaurus. 



In comparing the relative potential intelligence of the two, one has also to bear in mind the great 

 preponderance of the cerebrum over the other parts of the elephantine brain, while in Stegosaurus the 



1 Dinosaurs of North America, 16th Ann. Ttept. IT. s. Geol. Surv., pt. 1, 1896, pp. 187-18S. 



2 Amer. Joum. Sci., vol. 30, 1910, pp. 371-372, flg. 9. 



