PEEISSODACTYLA. 713 



articulation is faceted as in the hiorher ung-ulates, but lacks the inferior 

 condyloid face of those types. The completeness of the metapodial distal 

 keels is a feature of high special organization only seen in the Equida; of 

 this order The coossification of the external malleolus is also a character 

 peculiar to the Equidoa among the Perissodadijla. There are two other char- 

 acters which are not elsewhere found in this order, viz, tlie articulation of 

 the fibula with the calcaneum, and the absence of the vertebrarterial canal. 

 Tlie former belongs to the Artiodadyla generally, and to the Prohoscidea, 

 and the latter to the ruminant family of the Camelida. Tims the Macrau- 

 cheniidce stand out as one of the most distinct of the families of the Perisso- 

 dadi/ht, and one to which we may anticipate considerable accessions in 

 future. But two species of Ilacrcmchenur are known, a larger 31. pcda- 

 clionica, and a smaller, 31. hoUviensis, both from the Pliocene formation of 

 South America. 



MENODONTID^. 



The known genera of this family are not numerous. They are defined 



as follows: 



a. Last supei'ior premolar ouly with two inner tubercles. 

 Incisors present DipJacoclon. 



aa. All the superior i^remolars with two interior cusps. 



Six inferior incisors ; canines very large Dceodon. 



Six inferior incisors ; canines very small . - Menodus. 



No inferior, and four small superior incisors ; canines very small Symborodoti. 



The genera are, as yet, exclusively American. Diplacodon, in its sim- 

 pler premolars, approaches the Chalicotheriidce, and is the oldest of the 

 American genera. It is from the Uinta or Upper Eocene. 3Ienodns and 

 Sijmhorodon, which include some species of gigantic size, belong in the 

 White River or Oligocene, Avhile Dceodon has so far only been obtained from 

 the John Day or Middle Miocene. 



Up to the present time no species of this family have been described 

 from the American Middle Eocene 



PAL^OTHERIID^. 



This family has been already defined. In its complex premolar teeth, 

 which in the upper jaw resemble the molars in composition, it shows an 



