1910.] The Arliodactyla. 401 



carpus (Fig. 26, A') shows the same elements, arranged in much the same 

 manner, as in the oldest Perissodactyl. It is alternating in type, the centrale 

 is fused with the scaphoid, and the lunar has a broad contact with the 

 unciform; the third digit also is considerably longer than its fellows. Again 

 in the Tertiary Equidee the fore limb resembles that of the Ruminants in 

 the humerus and in the V-shaped scapula, vestigial ulna shaft and long 

 cannon bone; and in certain Miocene horses the proportions of the limbs 

 closely paralleled those of Odocoileus (Gidley, 1903). The so called Artio- 

 dactyl characters of Titanotherium are discussed below (p. 385), as well as 

 the general resemblance in the manus between ilfefam?/norfo?? and Hippo- 

 potamus (p. 109). 



Yet in spite of these resemblances the evidence indicates that the Dip- 

 lathra or Ungulata Vera is a wholly unnatural group, that the Artiodactyla 

 and Perissodactyla have been derived from very different sources, and that 

 the resemblances are either primitive protungulate characters or of an entirely 

 convergent nature. 



The differences between the oldest Eocene Perissodactyls and most 

 Artiodactyls are very striking. In the earliest Perissodactyls, e. g., Eohippus, 

 Hepiodon, and Eotitanops, the third digit of the manus is markedly longer 

 than its fellows and digits II and IV tend to be subequal; in the oldest Artio- 

 dactyl type the third digit is indeed longer than the fourth, but digits IT and 

 V tend to become subequal. In the pes the difference between the two 

 orders is still more pronounced. In all known Perissodactyls the pes is 

 functionally tridactyl and raesaxonic, the middle digit being much the longest ; 

 in the most primitive Artiodactyls the pes is tetradactyl and paraxonic. In 

 the Lower Eocene Perissodactyls (e. g., Eohippus, Hepiodon, Lambdo- 

 therium, Eotitanops) the cuboid facet of the astragalus is represented by a 

 very thin limited surface, the navicular facet is gently convex, and the 

 narrow sustentacular facet lies on the inner posterior border. In the oldest 

 Artiodactyls the astragalus has a very broad contact with the cuboid, the 

 navicular facet is sharply convex and the sustentacular facet is broadly 

 oval and more central in position. These characters were certainly estab- 

 lished by the Middle Eocene (Bridger). 



In the dentition the contrasts are equally marked. Even in the oldest 

 Perissodactyls the posterior premolars are already relatively large and com- 

 plex and rapidly assume the molar pattern; the molars also are quadrangu- 

 lar, the protocone being at the antero-internal corner of the crown and the 

 hypocone well developed. The lower molars are already acquiring well 

 developed proto- and metalophids or two subequal Vs. In the Artiodactyla 

 the premolars are from the first retarded in their development, p* is at most 

 bicuspid, p^ is compressed, the molars are still triangular and tritubercular 



