614 THE EOCE^^E FAUNA. 



PERISSODACTYLA. 



This, the fii"st subordei' of the Diplarthrous Mammalia naturally occu- 

 pies a position between the AmUypoda and the Artiodadyla. Its lower 

 forms are more specialized in the structure of the feet than the Amhlypoda, 

 while its highest types do not reach the perfection of structure seen in the 

 Artiodadyla. This is particularly indicated by the form of the astragalus, 

 which has but one, the tibial trochlea, and never displays the distal one 

 characteristic of the cloven -footed families. The Perissodactyla occupy, 

 as regards their dentition, a position parallel with the Artiodadyla. They 

 are alwa}"s su])erior in dental complication to the Proboscidia and the suil- 

 line Artiodadyla, but only one series, that of the horses, reaches the com- 

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

 the Perissodadyla might be described as intermediate between that of the 

 Proboscidia and the lowest selenodont Artiodadyla. 



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

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

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

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

 and certain suilline Artiodadyla, 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 4-4 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 United States Geological Survey of the Territories, 1879, p. 228. A 



modification in the diagnoses of the families Chalicotheriidce and Palceothe- 



riidce was introduced into a subsequent memoir on the classification of the 



order in the Proceedings of the American Philosophical Society for 1881: 



I. Anterior exterior cresceut of superior molar.s sbortened, not disting;aislied from 

 the posterior by external ridge; inferior molars with cross-crests; premolars 

 different from molars. 



1. Toes 4-3 LopMndoniidce. 



2. Toes 3-3 Triplopodidce. 



