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 

 flat 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 



Fig. 146. — Left pes of Tapir. i i c i • i j.t_ 



CaL, calcaneum. As., astrag- from othcr hoofed auimals, there are 

 aius. N., navicular. Cn. 1, j^^auy others which it is needless to 



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



third cuneiforms. Mr. II, III, enumerate. 



IV, second, third and fourth r^^ie subioincd table gives the 



metatarsals. 



families and principal genera of the 

 American Perissodactyla ; extinct groups are marked f. 



Suborder CHELODACTYLA. Normal Perissodactyls 



I. EquiDiE. Horses. 



^Eohippus, low. Eoc. ^Orohippus, mid. Eoc. ■\Epihippus, up. Eoc. 

 ^Mesohippus, low. Oligo. fMiohippu.^, up. Oligo. "fAnchithe- 

 riuni, up. Oligo. f Parahippus, low. Mioc. to low. Plioc. ^Des- 



