?x 



0. C. Marsh — Principal Characters of the Dinocerata. 165 



)ject downward and backward, showing that the head was 

 ;lined when in its natural position. The exoccipitals are per- 

 forated by a condylar foramen of moderate size, which is sep- 

 arated from the larger foramen lacerum posterius by a slender 

 partition of bone. Between the post-glenoid process and the 

 basi-sphenoid, there is an irregular cavity filled in part below 

 by the periotic. There is a distinct alisphenoid canal, and the 

 foramen ovale is near its posterior orifice. In front of its an- 

 terior opening, is a small foramen lacerum anterius, and further 

 forward, the optic foramen. The infraorbital foramen is large, 

 and partially concealed behind the maxillary ridge which sup- 

 ports the malar. 



The brain cavity in Dinoceras is perhaps the most remarka- 

 ble feature in this remarkable genus. It proves conclusively 

 that the brain was proportionately smaller than in any other 

 known mammal, recent or fossil, and even less than in some 

 reptiles. It was, in fact, the most reptilian brain in any known 

 mammal. In D. mirahih, the entire brain was actually so 

 diminutive that it could apparently have been drawn through 

 the neural canal of all the presacral vertebrae, certainly through 

 the cervicals and lumbars. The size of the entire brain as com- 

 pared with that of the cranium is well shown in the accompa- 

 nying cut, figure 1. 



The most striking feature .„._-_ 

 relatively small size of the cerebral fossa, this being but little 

 larger than the cerebellar portion. This is shown in Plate lY, 

 the figures of which are drawn from a cast of the brain cavity 

 of B. mirahile, the type of the genus. The cerebral hemi- 

 spheres did not extend at all over either the cerebellum or the 

 olfactory lobes. The latter were large, and continued well for- 

 ward. The hemispheres were apparently convoluted, and the 

 Sylvian fissure distinctly marked. There was a rudimentary 



