172 HIGH SCHOOL ZOOLOGY. 



fossil forms the fifth toe was also present in the fore foot, as in 

 the Tapir. The earlier fossil Rhinoceroses had a more com})lete 

 set of teeth than the living species, which have no canines, and 

 have a tendency to lose the incisors, while the molars are present 

 to the full number f . Like the Elephants, the Rhinoceroses had 

 once a much wider geographical distribution, for remains of the 

 woolly rhinoceros are found with those of the mammoth in 

 Siberia, and numerous American representatives of the family 

 have also been found. Among these are Miocene forms which 

 had two horns side by side, and attained elephantine dimensions 

 ( Brontotlierhmi ). 



28. The third family of Perissodactyla is represented at the 

 present day by the single genus Eqims, to which the horse and 

 various species of asses and zebras belong. It differs from the 

 foregoing, in that the body is supported entirely on the third 

 toe, the distal joint of which (coffin-bone) is covered with the 

 hoof, while the second joint (coronary) and proximal joint 

 (fettei"-bone), but especially the metapodials (cannon-bones) are 

 much elongated, bringing the wrist and ankle-joint up to the 

 middle of the leg (Fig. HOP). The second and fourth toes have 

 disappeai'ed almost entirely, but they are represented by the 

 rudimentary metapodials (splint-bones), the proximal ends of 

 which- only are complete. Occasionally it occurs that a horse 

 is born with, these rudimentary digits in a more perfect con- 

 dition, the splint-bones not only being complete, but carrying 

 short digits, the ends of which may be clad with miniature hoofs. 

 Such a three-toed horse is evidently a reversion to a more 

 primitive Perissodactyle type, and such reversions are known 

 as instances of '' atavism." 



This family presents a more typical dentition than does the 

 foregoing. The incisors ai-e |, the canines small, especially in 

 the mare, and there is a long diastema between the front teeth 

 and the grinders which number |, the first milk molar not 



