604 



HISTORICAL GEOLOGY 



the early Eocene (Wind River) and became extinct with apparent 

 suddenness in the early Oligocene (White River), just as it had, 

 perhaps, reached its greatest abundance and variety. 



Two groups of titanotheres are represented in the Lower Eocene 

 near the beginning of the history of the race; one abundant genus 

 (Lambdotherium) had slender limbs and was capable of swift move- 

 ment, indicating that it was adapted to the open basins of the moun- 

 tain regions ; and the other (represented by Eotitanops) was composed 



Fig. 544. — Evolution of the titanotheres. (After Scott.) 



of larger and stockier animals, ancestors of those in the Oligo- 

 cene, that grew to be about two thirds the size of a tapir. The 

 largest, as well as latest, forms (Brontotherium) were ambulatory 

 creatures with an elephantine body but with legs less massive than 

 those of an elephant. The head (Fig. 544) was saddle-shaped, with 

 a pair of large horns, placed side by side and branching off from 

 the end of the nose, and which were probably covered with a 

 callous skin. The brain was not larger than the fist of an average 



