ODD-TOED UNGULATES 



(PERISSODACTYLA). 



Hoofed animal mostly of large size, usually with an odd number of toes on both pairs of feet, the middle toe being 

 the one that continues the axis of the leg. The thigh-bone has a third trochanter; dentition complete; 

 stomach simple; teats abdominal or inguinal; placenta usually diffuse and composed of separate cotyledons 

 distributed over the whole surface of the ovum. 



The animals belonging to this order formed 

 the greater part of the division of the Pachy- 

 derms with an odd number of toes of Cuvier, 

 the group of herbivorous Pachyderms of 

 other authors, who do not ascribe so much 

 importance to the structure of the feet as has 

 been done in recent times especially with 

 respect to the fossil series. The genera now 

 living are, in fact, only the greatly thinned 

 and isolated relics of the fossil types, and in 

 order to acquire a proper understanding of 

 the relations of the living Perissodactyla 

 among one another, it is necessary to have 

 recourse to the fossil forms from which they 

 are derived. 



We comprise in this order the Rock-badger 

 or cony family (Hyracida), Rhinoceroses, 

 Tapirs, and Horses, but willingly confess that 

 the rhinoceroses and the tapirs alone have 

 near relations of affinity to one another in the 

 fauna of the present day, while the horses, 

 and in a still greater degree the rock-badgers, 

 appear to be much more divergent types. 



The leading character of this order consists 

 in the structure of the feet, which serve for 

 no other use than standing or running, and in 

 which the distal or lower end is dominated by 

 an axis running through the middle toe, to 

 which all the others are from the first sub- 



ordinate. We have at the present day not a 

 single five-toed perissodactyle surviving, but 

 the original number of the toes was five, and 

 if we trace the series back to the oldest Eocene 

 strata, we can demonstrate the existence of 

 certain five-toed perissodactyles in these strata, 

 and arrange the forms in stages, showing how 

 by a gradual reduction in the number of toes 

 we arrive at the apparently single-toed foot of 

 the horse. The law according to which these 

 reductions take place is easy to understand. 

 First of all the toes destined to disappear 

 do not develop sufficiently to touch the 

 ground, they become steadily shorter and 

 more rudimentary, while the middle toe gains 

 in importance, and is brought into the same 

 line with the bones belonging to it of the 

 carpus and metacarpus in the fore-limbs, or 

 tarsus and metatarsus in the hind limbs (that 

 is, the bones corresponding in the one case 

 to those of the wrist and palm of the hand, in 

 the other to the ankle and sole of the foot). 

 This process of reduction first affects the first 

 or innermost digit, which disappears before 

 all the others. The fore-feet of the tapirs 

 and rock-badgers still have four toes; the first 

 digit is altogether wanting, but it is at once 

 felt that the fifth digit is already condemned 

 to impotence and tends to vanish. This loss 



