62 



THE EVEN-TOED UNGULATES. 



have their metacarpal or metatarsal bones 

 still separable; and the bones of the limbs, 

 radius and ulna in the fore-limb, tibia and 

 fibula in the hind-limb, still persist as distinct 

 bones. This structure is that seen in the 

 pigs. Everyone knows that a pig's foot may- 

 be split along its whole length to the wrist or 

 ankle. The reduction in the toes advances 

 still further in the peccaries, that peculiar 

 family of American pigs. In one species the 

 inner toe of the hind-foot has vanished, so 

 that this foot is only three-toed, and the 

 metatarsal bones of the middle toes begin to 

 get fused at their upper ends. Step by step 

 we can follow in the same manner the reduc- 

 tion of the radius and fibula, that of the two 

 lateral toes, and the fusion of the principal 

 metacarpal and metatarsal bones, in the fossil 

 forms as well as in those now living. A 

 genus of living musk-yielding animals, Hyae- 

 moschus, which has also been found fossil, 

 still presents astonishing similarities to the 

 peccaries in the structure of its limbs. The 

 ulna, which in the other ruminants is reduced 

 to a rudiment, still exists entire and quite 

 separate from the radius. The metacarpal 

 bones are not fused, while in the hind-limbs 

 the tibia and fibula are completely fused and 

 the metatarsal bones united at their lower 

 end. The fusion goes on in such a manner 

 that in the other ruminants, as in the horses, 

 there comes to be only a single metacarpal 

 or metatarsal bone, which, however, is dis- 

 tinguished from that of the Solidungula by 

 always having more or less well-marked lon- 

 gitudinal grooves running down the middle 

 line both before and behind, indicating the 

 place of fusion of bones which are still sepa- 

 rate in the embryo. This bone carries only 

 the two middle toes, and at its lower end 

 shows two rounded joint-surfaces correspond- 

 ing to them. The two side toes gradually 

 disappear, but are still represented externally 

 by prominences and so-called false or acces- 

 sory hoofs, and are recognizable in the skele- 

 ton by small style-like bones. Finally, in the 



giraffe the limbs have reached the last stage 

 of simplification: there is neither ulna nor 

 fibula; the metacarpals and metatarsals are 

 simple, without any visible groove or any trace 

 of lateral toes. It will be seen that the process 

 of simplification in this case is similar to that 

 which we observed in the Perissodactyla, 

 with only this difference, that in these there 

 was only a single dominant toe, while in the 

 Artiodactyla the changes all took place in 

 relation to two toes of equal importance. 



We do not intend to enter into details re- 

 garding the modifications which the different 

 bones of the limbs undergo, but must, never- 

 theless, remark that the astragalus, the bone 

 which is so characteristic in the ankle of the 

 hind-foot, has a very different form from that 

 which it presents in the Perissodactyla, so 

 that we can say at the first glance whether 

 this bone belongs to a member of the one 

 order or the other. 



The dentition likewise presents remarkable 

 transitional series. Originally all these ani- 

 mals had forty-four teeth in all, three incisors, 

 one canine, and seven cheek-teeth in each half 

 of the jaw, and this number has only got re- 

 duced at a comparatively late period; for even 

 in the ruminants, which have neither upper 

 incisors nor canines, there are found in the 

 embryos the germs of these teeth, which never 

 get developed. In an Eocene artiodactyle, 

 the Anoplotherium, we even find all the teeth 

 in continuous close-set rows of equal height, 

 as in man. The cheek-teeth are always com- 

 plex in structure; composed of eminences or 

 wrinkled and folded tubercles as in our pre- 

 sent pigs, forming the type of the Bunodontia, 

 or, as in our ruminants, of half-moon-shaped 

 enamel folds, forming the type of the Seleno- 

 dontia. Now the specialization of the teeth 

 goes on hand in hand both with their reduction 

 in number and with the change in the habits 

 of the animals from an omnivorous to a purely 

 vegetable diet, which change again is con- 

 nected with the conversion of sturdy thickset 

 forms into slender ones and with the loss or 



