382 



LAND MAMMALS IN THE WESTERN HEMISPHERE 



mously inflated and the orbits and ear-openings raised, pre- 

 sumably in adaptation to an amphibious mode of life. These 

 were the extremes of change within the family ; the other phyla 

 need not be considered. 



(4) At an early stage the digits were reduced from five to 

 four, first in the pes and then in the manus, and there reduction 

 ceased ; though in ^Merychyus, especially in the upper Miocene 



species, the lateral 

 digits were very 

 slender and, had 

 this series survived, 

 it would probably 

 have led to didactyl 

 forms. 



In other respects 

 there was very little 

 difference in the 

 skeletons of the vari- 

 ous phyla and herein 

 lies the peculiarity 

 in the history of the 

 family, great variety 

 in the form of the 

 skull, and, relatively speaking, hardly any change in the body, 

 limbs or feet. In the horses, rhinoceroses and ftitanotheres 

 the modifications of the successive genera affected all parts of 

 the structure, but in the foreodonts, except for the loss of one 

 digit in manus and pes and variations in the length of the tail, 

 the skeletons of the latest genera did not differ in any impor- 

 tant respect from those of the earliest. Such a combination 

 of mutability and plasticity in the skull with extreme conserva- 

 tism in the remainder of the bony structure is an exception to 

 the usual mode of development, though something of the same 

 sort has already been pointed out in the case of the tapirs 

 (p. 325) and will recur in that of the elephants (Chap. X). 



Fig. 204. — Left manus of toreodonts. A, ^Merycoido- 

 don culbertsoni, White River. B, ^Merycochcerus 

 proprius, upper Miocene. 



