508 THE WASATCH AND BRIDGER FAUNiE, 



external two metacarpals on its distal face. This lengthening- of the navic- 

 ular is a specialty of the Proboscidea among hoofed Mammals, the shorter 

 form being characteristic of the lower types of both Perissodactyla and 

 Artiodactijki, where the astragalus has two distal articulations. In the 

 Perissodactyla, the extent of the navicular increases until the highest genus, 

 the Horse, is reached, where it almost covers the entire end of the astraga- 

 lus; but, in the Artiodadyla, the extension of the cuboid over the astragalus 

 does not diminish. The nearest approach to the distal articulation of the 

 astragalus of the Amblypoda, outside of the order, is seen in the Miocene 

 Perissodactyle genus Symborodon. Here the cuboid and nlivicular facets 

 are flat, and are separated by an oblique line, so as to be similarly incapable 

 of hinge-like movement. The resemblance to the lowest Artiodadyla {e. g., 

 Oreodon, Hippopotamus) is very remote, for there the two facets are parallel, 

 offering a ginglymus to the articulating bones. 



The difference between the fore foot of the Amhlypoda and that of the 

 Proboscidea consists in the alternating position of the elements of the two 

 carpal rows. This is also a character of the two other living orders of 

 hoofed Mammals, and maintains itself with great persistency in both of 

 them. It is essentially a primitive character, the alternating position being 

 usual in the cold-blooded Vertebrata, and is the pereistence of the oblique 

 relation of the original divergent branching rays, to which digits have been 

 traced. In the Proboscidea and Hyracoidea, the elements of the two rows 

 assume an opposite and longitudinal relation. The stnicture of the fore 

 foot in the Amblypoda appears to be about equally related to that of the 

 Proboscidea, the Perissodactyla, and the Artiodactyla. 



In the cubito-carpal articulation, the resemblance is again to the 

 Proboscidea in the relatively large proportion of it belonging to the ulna, 

 and the consequent lateral position of the latter bone. In this respect, it 

 differs much more from the other two living orders of hoofed Mammals, 

 although here again the lower forms of both resemble the Amhlypoda more 

 than do the higher forms. As is well known, both of the hoofed orders 

 display a constantly diminishing extent of the ulno-carpal aiticulation, and 

 increase of the radio-carpal, until, in the Horse and Ox, the ulna becomes 

 a mere splint attached to the radius. 



