EQUUS. 469 



broad hoof. This single digit is the third (not, as was formerly 

 thought by some naturalists, two toes united), the rudimentary 

 metatarsals and metacarpals of the second and fourth dibits form- 

 ing the splint bones, one on each side *. The ulna and fibula are 

 rudimentary and incomplete distally. 



The general form is graceful, and the limbs are adapted for 

 great speed. The head is elongate, there is a crest or mane of 

 longer hairs along the back of the neck, and there are long hairs 

 on the tail. Inside each forearm in all living species, and also 

 inside each tarsus in the horse (E. caballus) only, is a peculiar 

 callosity, the use of which is not known. There are two inguinal 



Dentition : i. J, c . ^, pm. |-|, m. gj. Canines generally want- 

 ing in females. There is sometimes an additional small anterior 

 upper premolar. The incisors have a flat crown, with at first a deep 

 hollow in the middle; this (the "mark" in horses) disappears with 

 age. The premolars and molars have flat rectangular crowns with 

 extremely complicated folds of enamel, and are of the hypsodont 

 type, having elongate crowns and short roots. Vertebrae : C. 7 

 D. IS, L. 6,8.5,0.15-18. 



Fig. 152. Crowns of (a) upper and (b) lower second right true molars of 

 Equus hemionus, the inner side uppermost. 



This genus contains the horses, asses, and zebras, now restricted, 

 in the originally wild state, to Asia and Africa, though wild horses, 

 descended from tame animals, abound in parts of America. One 

 species occurs on the north and west frontiers of India. 



In late Tertiary times the genus had a far wider range, and re- 

 mains of several species are found in Indian deposits. Two forms, 

 one indistinguishable from E. asinus, the other closely resembling 

 E. Jiemionus, are represented in the Karnul Caves (Pleistocene) ; a 

 larger kind, E. namadicus, in the Pleistocene Nerbudda beds ; and 

 two species of Equus, besides four of the 3-toed Hipparion, in the 

 Pliocene Siwaliks. 



* In several extinct genera of Equidte other digits were developed, and a 

 gradual passage from a four-toed form to the present greatly specialized single- 

 toed type has been traced. 



