PEEISSOD ACT YL A . 



615 



II. Exterior crescents of superior molars as in I; inferior molars with cross-crests; 

 superior molars and premolars alike, with cross-crests. 



3. Mastoid bone forming part of the external wall of the skull Mgracodontidce. 



4. Mastoid bone excluded from the walls of the skull by the contact of the occipital 



and squamosal Rhinocericla: 



III. Exterior crescentoid crests of superior molars subequal, distinct; mferior 



molars with cross-crests. 



5. Superior molars and premolars alike and with cross-crests ; toes 4-3 Tapiridw. 



IV. The external crescentoid crests of the superior molars subequal, separated by 



an external ridge; inferior molars with crescents. 



A. Superior premolars different from molars; with only one internal cusp. 



6. Toes 4-3; a vertebrarterial canal ChaUcothcriida: 



7. Toes 3-3; no vertebrarterial canal Macrauclieniidce. 



A A. Premolars like molars, with two internal lobes above. 



8. Toes with digits, 4-4 Menodoiitidce. 



9. Toes with digits, 3-3 Palccothcriida: 



10. Toes with digits, 1-1 Equidce. 



The genera included in these families are the following. The table 

 shows their geological distribution: 



Lophiodontida;. 

 Systemodon Cope 



Hyracotherium Ow 



Plioloplius Ow 



Heptodon Cope 



Helaletes Marsh 



X,ophiodon Cuv 



Hyiacliyus Leidy 



Colonoceras Marsh 



Tri2)lopidce. 

 Triplopus Cope 



Hyracodontidce. 

 Hyracodon Leidy 



EhinocerontidcE. 

 Orthocynodou S. aud 



Aceratherium Kauj) 



Ccenopiis Cope 



Diceratherium Marsh 



Zalabia Cope . . . 



Aphelops Cope 



