336 



ORDERS OF MAMMALIA. 



ill the number of the digits. In all, however, the third 

 digit is pre-eminently and symmetrically developed, and the 

 foot is distinctly of the Perissodactyle type, wliile the femur 

 has a third trochanter. There is a discontinuous series of 

 teeth in each jaw — the prsemolars being always separated 

 by a wide diastema (fig. 636) from the teeth in front of 

 them — and canines are usually present in the males only. 

 Tlie dental formula is — 



3—3 



or 



0—0 

 0—0 



3- 



j^m 



3—3 

 m =40. 



The incisors are of moderate and equal size in l)oth jaws, 

 and typically {Equus and Hipixirion) exhibit a deep pit in 

 their crowns ; the canines of the males are usually well 

 developed, but not tusk-like ; and the molars are extremely 

 long. The outer side of the molars is deeply grooved with 



Fig. 636.— Skull of the Horse {E(pius mhallas). 



two parallel grooves, the outer wall being thus doubly 

 crescentic in transverse section, while there are two internal 

 ridges corresponding with the external crescents. The val- 

 leys separating the crescents are filled up with cement, with 

 which the external surface is also coated. The lower molars 

 are doubly crescentic, and present resemblances to the cor- 

 responding teeth in the Pcdcwtheria. 



As regards the range of the Equidcc in time, the earliest 



