8 THE HORSE AND ITS RELATIVES 



dividing the third from the fourth toe, whereas 

 in the odd-toed group it is constituted by a line 

 running down the middle of the third toe. 



This, although by far the most important, is, 

 however, by no means the sole character in which 

 the skeleton of an odd-toed differs from that of an 

 even-toed ungulate. One of the most easily recog- 

 nised of these minor skeletal differences is the 

 presence in the members of the odd-toed group 

 of a more or less strongly marked projection or 

 process on the outer side of the upper end of the 

 shaft of the femur, or thigh-bone, which is totally 

 absent in the even-toed group. Another, although 

 less obvious, difference is to be found in the shape 

 of the astragalus, or huckle-bone, of the tarsus, or 

 ankle-joint — the so-called hock of the horse. In the 

 Perissodactyla the lower surface of this bone is 

 markedly flattened, whereas in the Artiodactyla the 

 same surface is rounded and pulley-like : the vertical 

 diameter of the whole bone being also relatively 

 less in the former than in the latter group. Yet 

 another difference is to be found in the number 

 of joints, or vertebrae, in the backbone, or vertebral 

 column, of the two groups. In perissodactyles the 

 number of vertebrae between the skull and what 

 is known as the sacrum (that is to say, the con- 

 solidated mass of vertebrae to which the haunch- 

 bone, or pelvis, is attached) is never less than 29 and 

 is very generally 30, whereas in the artiodactyles 



