1887) | The Perissodactyla. l 1001 
_ know how to separate it from Amynodon. Metamynodon and 
_ Hyracodon are from the White River Miocene. The family 
_ is not yet known from higher formations, and appears to have 
died out. Why such a robust and well-defended type as Meta- 
_mynodon should have disappeared, and the comparatively weak 
_ and entirely unarmed Czenopus should have survived, is not easy 
to understand. In Hyracodon we observe a degeneracy of the 
_ anterior dentition of both jaws. According to Scott, Hyracodon 
nebrascensis was “a slender, long-limbed, and slightly-built ani- 
mal, with a long neck.” Three or four species only are known. 
The entire family is, so far, only known from North America. 
The phylogeny may be thus represented: 
Metamynodon. Hyracodon. 
Amynodon. 
, 
“sem 15. Metamynodon planifrons S. and O.; skull, one-sixth natural size; ge 
From Scott and Osborn, in “ Bulletin of Mus. Compar. Zodlogy Cambridge. 
iue y. The general relation of the component parts of the | 
tide molars in the rhinoceros family is that of the Lophiodon- 
~~ ‘tall important respects. The external ends of the cross- 
