290 



LAND MAMMALS IN THE WESTERN HEMISPHERE 



foot is usually odd, 1, 3 or 5, but four- 

 toed forms occur, as the tapirs, which 

 have four toes in the front foot, three 

 in the hind ; the important character 

 is that the median plane of the foot 

 bisects the third digit, which is sym- 

 metrical. The third and fourth, each 

 asymmetrical, together form a sym- 

 metrical pair. Especially character- 

 istic is the form of the astragalus and 

 calcaneum (ankle and heel bones) ; the 

 astragalus has but a single, deeply 

 grooved and pulley-like surface, that 

 for the.tibia, the lower end is nearly 

 fiat and rests almost entirely upon the 

 navicular, covering but little of the cu- 

 boid (see Figs. 146, 148). The cal- 

 caneum does not articulate with the 

 fibula and its lower end is broad and 

 covers most of the cuboid. 



While the foregoing list includes 

 the most important of the structural 

 features which are common to all 

 perissodactyls and differentiate them 

 from other hoofed animals, there are 

 many others which it is needless to 

 enumerate. 



The subjoined table gives the 

 families and principal genera of the 

 American Perissodactyla ; extinct groups are marked t- 



M. 



Fig. 146. —Left pes of Tapir. 

 Col., calcaneum. As., astrag- 

 alus. N., navicular. Cn. 1, 

 Cn. 2, Cn. 3, first, second and 

 third cuneiforms. Mr. II, III, 

 IV, second, third and fourth 

 metatarsals. 



Suborder CHELODACTYLA. Normal Perissodactyls 



I. Equid^. Horses. 



^Eohippus, low. Eoc. fOrohippus, mid. Eoc. ^Epihippus, up. Eoe. 

 fMesohippus, low. Oligo. ]Miohippus, up. Oligo. ^AruMhe- 

 rium, up. Oligo. t Parahippus, low. Mioc. to low. Plioc. ^Des- 



