DIPLAETHEA. 613 



DIPLARTHRA. 



Cope; Proceeds. Amer. Philo. Soc, 1882, p. 444. Paleontological Bulletin, No. 35, p. 444 (Nov. 11, 1882). 



This order includes the two suborders of Perissodactyla and Artiodac- 

 tyla, as ah-eady explained (page 378). It is the Ungulata of some authors, 

 e. g., Professor Gill, but I have used this term in a wider sense in imita- 

 tion of other writers. 



The two suborders which compose the order Diplarthra are distin- 

 guished in their entirety by but few characters; but these are universal, 

 and easily observed. The constitution of the carpus and tarsus is essen- 

 tially the same in both, but the tai'sus of the Artiodactyla is more perfectly 

 adapted for rapid movement than that of the Perissodactyla.' To the 

 former order belong the species having the swiftest and most graceful 

 movement. Besides the inferior ginglymus of the astragalus, the Artio- 

 dactyla are characterized by equal development of the third and fourth 

 digits instead of the preponderating development of the third. But in some 

 members of the order, the third and fourth digits are not entirely equal; 

 the posterior foot of Eurytherium has three toes. In the Perissodactj^la the 

 third and fourth anterior toes of Menodiis are nearly equal. In some mem- 

 bers of the latter order (Hyracofherimii) the distal facets of the astragalus 

 are somewhat convex, so approaching the Artiodactyla. On these and 

 similar grounds, I think these two groups are scarcely to be distinguished 

 as orders, and it is highly proljable that future paleontological discoveries 

 will approximate their early representatives still more closely. 



The Perissodactyla abounded in Eocene time, and have diminished in 

 numbers to the present period, when but few species remain. They 

 appeared in the Wasatch Eocene, in North America, since none have yet 

 been discovered in the Puerco epoch. The Artiodactyla are not certainly 

 known from the Puerco, and are rare in all the later Eocene beds. They 

 begin to be abundant in the Miocene, and culminated in the Pliocene and 

 recent epochs. 



I have been aided in the investigation of this order by the work of 

 Kowalevsky, printed in 1873, "Monographic der Gattung Anthracotherium 

 Cuv. und Vei'such einer natiirlichen Classification der fossilen Hufthiere." 



iPor a discussion of the question of the origin of the characters of these orders, see my paper 

 " On the effect of impacts and strains on the feet of Mammalia," American Naturalist, 1881, p. 542. 



