188 DINOSAURS OF NORTH AMERICA. 



The contrast in tbe 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 approxi- 

 mately 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 T ^g that of the alligator, if the 

 weight of the entire animal is brought into the comparison. If the 

 cerebral regions only of the two brains were compared the contrast 

 would be still more striking. This comparison gives, of course, only 

 approximate results, and some allowance should be made for the pro- 

 portionally larger brain in small animals. 



The brain of Stegosaurus ungulatus is clearly of a lower type than 

 that of Morosaurus, which, as the writer has shown, was several times 

 smaller in diameter than the neural canal in its own sacrum. In the 

 latter genus the brain was proportionally shorter, and the cerebral 

 region better developed, as shown in the plate cited. The absolute size 

 of this brain as compared with that of Stegosaurus is about 10 to 10, 

 the brain of the alligator figured being regarded as 1. Taking again 

 the body of the alligator as the unit, and Stegosaurus as 1,000, that of 

 Morosaurus would be about 1.500. Stegosaurus had thus one of the 

 smallest brains of any known land vertebrate. These facts agree fully 

 with the general law of brain growth in extinct mammals and birds as 

 made out by the writer. 1 



THE ANTERIOR VERTEBRAE. 



The vertebrae of Stegosaurus preserved all have the articular faces 

 of their ceutra concave, although in some the depression is slight. 

 They are all, moreover, without pneumatic or medullary cavities. Ou 

 PI. XLV a selection from the vertebral series of one skeleton is given, 

 which shows the principal forms. Figs. 1 and 2 represent a median 

 cervical. The other neck vertebra} have their centra of similar length, 

 but the diameter increases from the axis to the last of the series. Some 

 of the anterior cervicals have a small tubercle in the center of each end 

 of the centra, a feature seen also in some of the caudals. All the cer- 

 vicals supported short ribs. 



The dorsal vertebras have their centra rather longer and more or less 

 compressed. The neural arch is especially elevated. The neural canal 

 is much higher than wide. The head of the rib fits into a pit on the 

 side of the neural arch. Figs. 3 and 4 of PI. XLV represent a poste- 

 rior dorsal with characteristic features. The ribs are massive, and are 

 strengthened by their form, which is T-shaped in trans verse section. 



'Odontornithes, a monograph on the extinct toothed hirds of North America: U. S. Geol Espl. 

 Fortieth Par., Vol. VII, pp. 8, 121, 1880 Dinocerata, a monograph of an extinct order of gigantic 

 mammals: ilon. XJ. S. Geol. Survey, Vol. X, Author's Edition, pp. 57-59, 1884. 



