222 Reviews — Prof. 0. C. Marshes Dinocerata. 



size of the brain-cavity, and its position, in the skull in the genus 

 Tinoceras, also, is represented in Figure 1 0. 



"The most striking feature in the brain-cavity itself is the rela- 

 tively small size of the cerebral fossa, this being but little larger than 

 the cerebellar portion. The cerebral hemispheres did not extend at 

 all over the cerebellum or the olfactory lobes. The latter were large, 

 and continued forward. 



" The nerves passing off from the brain were large, and can be 

 made out with reasonable certainty. The olfactory lobes were sepa- 

 rated in front by an osseous septum. 



" In the genus Tinoceras, the brain was similar in its general 

 characters to that of Dinoceras, but appears to have been somewhat 

 more highly developed. The hemispheres were more elongate, and 

 the olfactory lobes relatively smaller. 



Brain- Groioth. — "The Dinocerata are by far the largest of all 

 known Eocene animals, and that they have, also, a very diminutive 

 brain is a noteworthy fact, which attracted the author's attention soon 

 after their discovery. 



"The comparison of the brain in this group with that of other 

 mammals from the same formation soon showed that the Dinocerata, 

 although most remarkable in this aspect, were not alone in diminu- 

 tive capacity of brain power. A more extended comparison led to 

 the fact that all the early Tertiary mammals had very small brains. 



" The results of this investigation were embodied by the author in 

 a general law of brain-growth in the extinct mammals throughout 

 Tertiary time. This law, briefly stated, was as follows : — • 



"1. All Tertiary mammals had small brains. 2. There was a 

 gradual increase in the size of the brain during this period. 3. 

 This increase was confined mainly to the cerebral hemispheres or 

 higher portion of the brain. 4. In some groups, the convolutions 

 of the brain have gradually become more complex. 6. In some, the 

 cei'ebellum and the olfactory lobes have even diminished in size. 

 6. There is some evidence that the same general law of brain- 

 growth holds good for Birds and Eeptiles from the Cretaceous to 

 the present time.^ 



" The author has since continued this line of investigation, and 

 has ascertained that the same general law of brain-gi'owth is true for 

 Birds and Reptiles, from the Jurassic to the present time." 



The small size of the brain in early Tertiary mammals will be 

 indicated by an examination of the Dinocerata skulls, with the brain 

 in position, shown in Figures 4, 5, 10, and 11. 



Prof. Marsh has still further illustrated this most interesting sub- 

 ject by giving in his Monograph figures of the crania of Limnohyus 

 robustus, Marsh (Middle Eocene) ; Amynodon advenus, Marsh (Upper 

 Eocene) ; Elotlierium crassum, Marsh (Miocene) ; Mastodon Ameri- 

 cnnus, Cuvier (Pliocene) ; and Platygonus compressus, Le Conte 

 (Pliocene), and many others, showing a well-marked advance in size 



^ See Silliraan's American Journal of Science, vol. viii. p. 66, July, 1874; and 

 vol. xii. p. 61, July, 1876 ; also Odontornithes, p. 10, 1880. 



