Cope.] ^ ♦ 8 [April 15, 



plexity of molars general in the Buminantia. The dentition of the mass 

 of the Perissodactyla might be described as intermediate between that of 

 the Proboscidia and the lowest selenodont Artiodactyla. 



The families of this order form a closely connected series, and the divi- 

 sion of them into three divisions, the " Pachydermata, " "Solipeda" and 

 Perissodactyla, has no warrant in nature. Especially unnatural is the con- 

 junction of the genera included under the first name, with the Proboscidia 

 and certain suilline Artiodactyla, in a single order, as was proposed by 

 Cuvier. The modifications of dentition from the simple type seen in 

 Menodus, to the most complex, as in Equus, are close and consecutive. 

 So, also, the gradual diminution in the number of digits from 5-4 to 1-1 

 can be traced through all the intervening stages. 



The following definitions of families are applicable in the present stage 

 of knowledge. Those of all but three were published in the Bulletin of 

 the U. S. Geological Survey of the Territories, 1879, p. 328. A modifi- 

 cation in the diagnoses of the families ChalicotlieriidcB and PalcBotTieriidm 

 is now introduced : 



I. Anterior exterior crescent of superior molars shortened, not distin- 

 guished from the posterior by external ridge ; inferior molars with 

 cross-crests ; premolars diflTerent from molars. 



1. Toes 4-3 Lophiodontidm. 



3. Toes 3-3 Triplopodidce. 



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

 cross-crests ; superior molars and premolars alike, with cross-crests. 



8. Mastoid bone forming part of the external wall of the skull 



Hyracodontidoe. 



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

 occipital and squamosal Bliinocerid(B. 



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

 inferior molars with cross-crests. 



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



Tapiridm. 



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 Chalicotheriidm. 



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



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



8. Toes with digits, 4-3 Menodontidm. 



9. Toes with digits, 3-3 PalcBotheriidce . 



10. Toes wiih digits, 1-1 Equidm. 



