THE FORERUNNERS OF THE HORSE 245 



the leading features of the genus Equus, which 

 includes the largest, and doubtless the swiftest, 

 members of the entire group, all of which are fitted 

 for a life on the open plains, where they subsist 

 entirely by grazing. In correlation with this kind 

 of life is the ofreat lens^th and columnar structure 

 of the cheek-teeth, which show an intricate enamel- 

 pattern on the grinding surface, and are characterised 

 by the union of the antero-internal pillar of those 

 of the upper jaw with the main body of the crown 

 by means of a narrow neck, as shown at A of 

 the figure on page ^t, ; the hollows between the 

 enamel-foldings being completely filled with cement. 

 In the skull the socket of the eye is surrounded by 

 a complete ring of bone, there is no deep depression 

 or pit immediately in front of the same, and the slit 

 between the fore part of the upper jaw and the 

 nasal bones is short. A long gap, or diastema, in 

 the front half of which are implanted the tusks, or 

 canines (when these are present), separates the 

 incisor from the cheek-teeth ; and the crowns of 

 the incisors themselves are penetrated, in early 

 life, by the pit, or "mark," which has already been 

 sufficiently described. Each limb terminates in a 

 single hoof, upon which alone the animal walks ; 

 but the lateral toes — second and fourth — of the 

 extinct three -toed members of the family are 

 represented by splints, which may either remain 

 free or be welded to the adjacent cannon-bone. 



