THE HOOFED MAMMALS. 127 



under which title are included not only horses proper, but also zebras and 

 asses, all of which may be comprised in the single genus 

 Equus. From all other living Mammals the members of The Horse Tribe. 

 this genus differ by the reduction of the number of toes to -Family 



a single one in each foot ; but as there are certain extinct Equidce. 



horses provided with three perfect toes on each foot, we 

 learn that this essential peculiarity of the existing forms is a feature of com- 

 paratively late acquisition. Indeed, evidence of this descent from a three- 

 toed ancestor is afforded by the so-called splint-bones which are found in the 

 horse, lying on each side of the upper half of each cannon-bone, and corre- 

 spond to the metacarpals and metatarsals of the second and fourth digits of 

 the typical five-toed foot, the cannon-bone representing the third or middle 

 one. In the case of such well-known animals as the horses, it would be 

 quite superfluous in a work of the present nature to describe them in any 

 detail, and it will accordingly suffice to point out a few of the features which 

 indicate that they form a family by themselves. More important than the 

 single digit of the feet is the peculiar structure of the molar and premolar 

 teeth, which form tall quadrangular prisms, in which the enamel is thrown 

 into a number of deep foldings and plications, the intervening depressions 

 and flutings being completely filled with cement. Although the resemblance 

 is at first not very easy to make out, a careful study of the pattern on the 

 crowns of the upper molar teeth of a horse will show that it is really essen- 

 tially the same as in the rhinoceroses, of which it may be regarded as a 

 specialised modification. The upper premolar teeth, which are generally 

 three in number, although occasionally a small anterior one is present, are as 

 complex as the molars, and are peculiar in being larger than the latter ; 

 similar features occurring in the lower jaw. There are thus normally six 

 pairs of cheek teeth in each jaw ; the total number of teeth in the adult male 

 being 40, although in the female it may be reduced to 36, as the canines, or 

 tusks, which are always rudimental in that sex, are in some cases altogether 

 wanting. The canines occupy the centre of a long gap between the pre- 

 molars and the incisors ; the three pairs of the latter forming a semi-circle 

 at the extremities of the jaws. The incisors of the horses are peculiar 

 in having the summits of their crowns deeply infolded, like the finger of 

 a glove with the tip pushed in ; and it is according as to how much of this 

 infold, or "mark," remains in the teeth of a horse that its age is approxi- 

 mately determined. The skull of a horse, which is of an exceedingly elon- 

 gated form, differs from that of either a tapir or a rhinoceros in having the 

 socket of the eye completely surrounded by a ring of bone ; and in the limbs 

 the bones known as the ulna in the front pair, and the fibula in the hind, are 

 incomplete, and respectively united with the radius and the tibia. A special 

 feature of the horses is the great elongation of the cannon-bone (metacarpal 

 and metatarsal) in each foot, which gives them their characteristic length and 

 slenderness of limb, and enables the upper parts of both the fore and hind- 

 legs to be enclosed in the skin of the body. It is almost superfluous to 

 observe that the so-called knee of a horse represents the human wrist, and 

 the hock the ankle ; the whole of the limbs situated below these joints corre- 

 sponding to the middle finger or toe of the human hand or foot, with the sup- 

 porting metacarpal or metatarsal bone. The toe of each foot is enveloped in 

 a solid hoof, which is broader in the front than in the hind-limb ; and the 

 inner sides of the fore-limb always has a naked wart-like callosity above the 

 wrist-joint, while there may be a similar pair of callosities on the hind-limb. 



