200 THE MAMMALIA. 



of the Rhinoceros species, shows a pair of bony 

 protuberances (the supports for mighty horns) on 

 the upper jaw, in front of the eye-cavities, and prob- 

 ably the nasal bones and the intermediate space 

 between the horns permitted the addition of a 

 proboscis. 



Both in the case of Brontotherium as well as 

 in some members of the family of the gigantic 

 Dinocerata, to be spoken of presently, the relative 

 size of the brain to the skull is known from fossil 

 impressions. According to these the size of the 

 actual brain substance must have been extremely 

 small (Fig. 34, B.). Its extent reminds one of the 

 relative proportions of the reptile brain, and points 

 to an incongruity which must certainly have had 

 its effect upon the dying out of this and of similar 

 species. It was in this manner that all the huge 

 reptiles of the middle geological periods became 

 extinct, especially as land animals. The few huge 

 but small-brained reptiles of the present day, such as 

 the crocodiles, clearly owe their existence to the fact 

 that they have continued to live in water, also to 

 their marked stability. A transition to life on land 

 would lead to their extinction. 



From the circumstance that one of the more 

 recent strata of Oregon contains the remains of a 



