THE SKELETON IN MAMMALIA. THE MANUS. 507 



have their ungual phalanges flattened on their contiguous 

 surfaces. 



In all ARTIODACTYLA the third and fourth digits are large, but 

 a gradual reduction in the second and fifth can be well traced. 

 Thus in the Suina the second and fifth digits, though smaller 

 than the third and fourth, are well developed and all four 

 metacarpals are distinct. In the Tragulina too all four meta- 

 carpals are developed, and in Dorcatherium the third and fourth 

 commonly remain distinct as in the Suina. In the other Artio- 

 dactyla however the third and fourth metacarpals are almost 

 always united, though indications of their separate origin 

 remain. In some Ruminantia, such as many Deer, the second 

 and fifth digits are reduced to minute splint bones attached 

 to the proximal end of the fused third and fourth metacarpals, 

 and to small hoof-bearing phalanges, sometimes attached to 

 splint-like distal vestiges of the metacarpals, sometimes alto- 

 gether unconnected with any other skeletal structures. In 

 some other Ruminants, such as the Sheep and Oxen, the only 

 remnants of the second and fifth digits are nodules of bone 

 supporting the hoofs, and in others, such as the Giraffe, Ano- 

 plotherium commune, some Antelopes and the Tylopoda, all 

 traces of these digits have disappeared. The Camels differ 

 from all living Ungulata vera in not having the distal pha- 

 langes completely encased in hoofs, and from all except the 

 Hippopotami in placing a considerable amount of the manus 

 on the ground in walking. 



While the manus of the Artiodactyla is symmetrical about 

 a line drawn between the third and fourth digits, that of the 

 PERISSODACTYLA is symmetrical about a line drawn through the 

 middle of the third digit, which is larger than the others and 

 has its ungual phalanx evenly rounded and symmetrical in 

 itself. The most reduced manus in the whole of the mam- 

 malia is found in the Horse and its allies, in which the third 

 digit, terminated by a very wide ungual phalanx, is the only 

 one functional. Small splint bones representing the second and 



