318 



LAND MAMMALS IN THE WESTERN HEMISPHERE 



very much lengthened, so as to form a hump at the shoulders, 

 and this elongation of the spines went on in proportion to the 

 growing weight of the head. 



(8) The limb-bones increased in thickness until they be- 

 came extremely massive, to carry the immense weight of the 

 body, and they eventually lost the marrow-cavities, which were 

 filled up with spongy bone, a great gain in strength. As is 

 generally, though not universally, true of the large and heavy 

 mammals, there was no coossification between the limb-bones 

 and no great increase in their proportionate length. The 

 thigh-bone, or femur, lost the cyhndrical shape of the shaft, 

 becoming flattened and very broad, and acquiring something 

 of the appearance of the same bone in the elephants. 



(9) There was no loss or coossification of elements in 

 wrist (carpus) or ankle (tarsus) and no reduction of digits within 

 the limits of the family. In the latest, largest and most special- 

 ized genera, as well as in the earliest, smallest and most prim- 

 itive, there were four toes in the front foot and three in the 

 hind. We have the most cogent reasons for assuming that all 

 mammals were derived from ancestors which had five toes in 

 each foot, neither more nor less. If this be true, then the most 



IF M M 



Fig. 166. — Right manus of t titanotheres. A., f Titanotherium, White River (after 

 Marsh). B., i Palwosyops, Bridger, Princeton University Museum. 



