50 



THE ODD-TOED UNGULATES. 



gated rhomboidal form. The cranial region 

 is relatively very small, forming only a little 

 more than one-third of the whole length of 

 the head, and it becomes greatly constricted 

 near the foramen viagmim or opening by 

 which the spinal column passes into the brain. 

 The brow is flattened and is continued almost 

 horizontally into the long nasal bones, which 

 form a beak-like roof above the nostrils. In 

 front of these projects the elongated portion 

 of the upper jaw which carries the incisors 

 and canines. The pretty large orbits are 

 surrounded by a complete bony ring, but 

 leave a wide opening behind into the temporal 

 fossae. The halves of the lower jaw are com- 

 pletely fused in the middle, as in the tapirs, 

 whose jaws have a great resemblance in their 

 general features to those of the Solidungula. 



The dentition of the horses also resembles 

 that of the tapirs in the general arrangement. 

 Both above and below we find a semicircle 

 composed of six incisors in the form of some- 

 what curved chisels with blunt edges. The 

 canines, which are in all cases but slightly 

 developed, and which in the females for the 

 most part disappear entirely, although more 

 prominent in the males, are larger in the 

 lower jaw than in the upper, and in that jaw 

 follow close after the incisors, while in the 

 upper they are rather farther apart from 

 these. As in all other Perissodactyla a long 

 interval divides the cheek-teeth from these 

 front ones. It is in this interval that the bit 

 is placed. The cheek-teeth, seven in number 

 in each half of each jaw, are much larger in 

 the upper jaw, where they present a quad- 

 rangular chewing surface, than in the lower, 

 in which these teeth are more elongated and 

 laterally compressed. The first of these 

 cheek-teeth is always very poorly developed, 

 reduced, in fact, to a mere stump; it is very 

 readily and often very early lost. The other 

 six cheek-teeth in the upper jaw appear as if 

 composed of two halves, which are soldered 

 together by a strong external vertical pillar. 

 To this external pillar there corresponds on 



the inner side another less prominent broad 

 pillar, which shows on the grinding surface 

 in the form of a loop of enamel. On the 

 surface of each tooth are to be seen four 

 sickle-shaped enamel stripes with their con- 

 cavity directed outwards; and these stripes 

 are separated from each other and in the 

 middle by deep fissures. In the lower cheek- 

 teeth the pillars are wanting. Each tooth 

 has on the outside and in the middle a deep 

 vertical groove, which indicates the separa- 

 tion into two halves, and the enamel stripes 

 run back each into itself, as if there had been 

 bosses or tubercles, the surface of which be- 

 came planed away. This kind of dentition 

 forms, as will be readily seen, an excellent 

 masticatory mill. 



The structure of the limbs deserves special 

 attention. The bones corresponding to the 

 upper arm and thigh are short, thick, and 

 buried in the flesh of the body. The third 

 trochanter on the thigh-bone is placed pretty 

 low down. The ulna and fibula are greatly 

 reduced, and are recognizable only as un- 

 shapely adjuncts at the upper part of the 

 radius and tibia respectively. The bones of 

 the wrist and ankle are greatly reduced both 

 in number and size. These are followed 

 below by a single bone, which is often errone- 

 ously spoken of as the shin-bone, but which 

 is nothing else than the excessively developed 

 metacarpal (or metatarsal) bone of the middle 

 toe, and has at its upper end two small 

 pointed style-like bones (the splint-bones) 

 attached to it on both sides, these being the 

 remains of the metacarpal (or metatarsal) 

 bones of the second and fourth digits, which 

 are thereby only indicated. Finally, at the 

 lower end of this elongated metacarpal or 

 metatarsal there come the three phalanges of 

 the middle digit, which form the fetlock and 

 hoof, the bone of which latter has pretty much 

 the same form as the horny hoof that covers 

 it. By studying the development it may be 

 shown that in the embryo three digits begin 

 to be formed, but that the two outer ones 



