74 



THE TWO-TOED ARTIODACTYLA. 



disappear altogether. The true cheek-teeth 

 are always formed of semi-cylinders, which, 

 through the disposition of the enamel, exhibits 

 on the chewing surface a half-moon, the con- 

 vexity of which in the upper cheek-teeth is 

 turned inwards and in those of the lower jaw 

 outwards. The selenodont (moon -shaped) 

 type of dentition is here accordingly developed 

 in all its purity. The premolars readily fall 

 into two groups. The first frequently stands 

 near the incisors and canines, so that it is 

 separated from the others by an interval or 

 diastema, while the originally simple hinder 

 premolars become associated with the true 

 molars both in position and form. Since the 

 function of the cheek-teeth consists specially 

 in the bruising of the grass by a lateral 

 grinding motion, the condyle, or joint-surface 

 at the end of the lower jaw, has the form of 

 a longish, transversely-placed cylinder. 



Even in some members of the hog family, 

 those, namely, belonging to Africa, we may 

 observe a tendency to the formation of bony 

 outgrowths on the skull, these having the 

 appearance of swellings. In the ruminants 

 we can trace step by step the growth of such 

 bony excrescences, which, according to their 

 structure, are called horns or antlers. The 

 original types of the ruminants had no horns, 

 which are likewise entirely wanting in some 

 still living families, as the camels and the 

 musk-deer. Three different forms can be dis- 

 tinguished in these weapons, which are often 

 developed only in the males, and, in any case, 

 are always stronger and larger in them than 

 in the females. In the giraffe a bony knob 

 rises in the middle of the brow a little behind 

 the eyes, and two short horns are formed just 

 at the back of the head between the ears. 

 They have bony cores which are connected 

 by sutures with the bones of the skull. All 

 these protuberances are covered by the quite 

 unmodified hairy skin. From this primitive 

 structure of a simple bony peg, so to speak, 

 covered with skin, have been developed, on 

 the one hand, horns, on the other, antlers. 



In the former we have a solid or hollow bony 

 core completely fused with the skull and 

 traversed by numerous vessels, the canals 

 through which these run giving a spongy 

 or striped appearance to the internal struc- 

 ture. This permanent core is covered by a 

 hard sheath, which, like the nails and hoofs, 

 is composed of horny fused fibres. These 

 horny sheaths keep constantly growing by 

 the addition of new layers internally. They 

 can easily be separated from the bony core, 

 with which they are connected only by vessels 

 and the soft tissue out of which the horny 

 substance is formed. Like the hoofs they 

 persist throughout life, and at the lower part, 

 where the bony core enters them, they are 

 hollow. It is the family of the Hollow-horned 

 Artiodactyla (Cavicornia) the members of 

 which are furnished with horns of that sort. 



The antlers of the Deer are formed in a 

 different way. From the hinder and upper 

 angles of the frontal bones there rise processes 

 or protuberances known as bossets, which 

 belong to the bone itself, and like it are 

 covered by the hairy skin. These processes, 

 usually very short, may attain, as in the case 

 of the muntjac, a considerable length, and 

 are manifestly analogous to the horns of the 

 giraffe. But in the deer they spread out at 

 the end into a disk surrounded by a ring of 

 bony knots forming the burr. On the disk 

 may be observed at certain times what is 

 nothing else than an inflammation, which 

 leads to the extraordinarily rapid growth of a 

 true bone traversed by numerous vessels and 

 covered by a thin layer of skin with short 

 hair. During the growth of this bony pro- 

 cess the number of blood-vessels is remarkable, 

 and the circulation of the blood in the grow- 

 ing bone is very active. But as soon as the 

 bone has attained its full length the circulation 

 gradually slackens, and ultimately it ceases 

 altogether. The skin becomes dry, breaks 

 off in fragments, and the whole antler dies. 

 It still remains for a time attached to the 

 burr, but finally it breaks off in order to give 



