Cope.] 378 [April 15, 
plexity of molars general in the Ruminantia. The dentition of the mass 
of the Pertssodactyla 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 
Perissodactyta, 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 Artéodactyla, 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 Hguus, 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. 228. A modifi- 
cation in the diagnoses of the families Chalicothertide and Paleothertide 
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 different from molars. 
1s T0@S'4=8 oo sees e cen diate oe e isadaisecar eve ees 4 Bares Lophiodontide. 
2. Toes 8-8. .......66. casa aybis apacsestantebssae tiSiecs ese aye Ganegege bes Triplopodida. 
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.......... 
Hyracodontide. 
4, Mastoid bone excluded from the walls of the skull by the contact of the 
occipital and squamosal............eeeeeeee tenees .... Bhinoceride, 
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... 
Tapiride. 
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 4-8; a vertebrarterial canal.................04. Chalicothertida. 
%. Toes 3-3; no vertebrarterial canal...............24. Macrauchentide. 
AA. Premolars like molars, with two internal lobes above. 
8. ‘Toes with digits, 4-8. .ecusisisesesseesdas svaiewwese cee Menodontide. 
9. Toes: With digits, 8-9 ssa se srcemda gees sa goes SESE Paleothertide. 
10. Toes with digits, 1-1.........0. 0. cece eee ee eee wig avereyiesatsacisia Equide. 
