2G0 
PERISSODACTYLA. 
teeth of Oligotomus and Orotlierkm, two genera nearly allied to Hyracotlie- 
rium, require but slight modification to present the forms' of later Perissodac- 
tyle genera. These are, on the one hand, Palceosgops and PalccoiJierium, 
where the inferior molars present two V’s, and on the other, Hyracliyus and 
Tapinis, where they present two cross-crests. In a diagrammatic scheme in 
this paper, I placed Oligotomus, Cope, in ancestral relation to the Equine Pe- 
rissodactyles on the one hand, and the Artiodactyles on the other. I have 
since shown that it is probable that the ancestral type of the Artiodactyles 
must be looked for in a genus possessing a less specialized character than 
Oligotomus, of the order Amhlypoda* 
The probable ancestral relation of Hyracotherkm iy’’ Orohippus’’) to 
Ancliitlierium and the Equine series was first asserted by the writer in a 
phylogenetic diagram published in the Annual Report of the United States 
Geological Survey of the Territories for 1872, issued early in 1873. In the 
text, the following language is used: “An interesting annectant form is seen 
in OroJiippus procyoninus, where the two intermediate tubercles which sepa¬ 
rate the inner cones from the outer V’s in Limnohyus, are so developed as to 
constitute parts of an incomplete pair of transverse ridges, which disappear 
in front of the bases of the outer V’s. They represent the oblique crests of 
Palmotlierkmi and Anchitlierium, and thus the genus Oroliippus (Hyracothe- 
rium) furnishes a station on the line from Palceosyops to the Horses.” Near 
the same time. Professor Marsh observes, under the caption of Oroliippus'^: 
“Additional specimens of this genus fully justify its separation from Anclii- 
therium, and likewise show that it holds a most interesting intermediate 
position between that genus and the less specialized Mammals of the Palceo- 
tJierium type.”t 
Later, f the writer traced the modifications in the structure of the molar 
teeth necessary in the course of descent from Hyracotlierium to Pquus. 
After quoting my comparison of the former genus with Anchitlierium, 
from the Annual Report above cited, I continue: “A greater longitudinal 
*Types of Molar Teeth of Mammalia Educabilia, March, 1874, p. 21; Relation 
of Man to the Tertiary Mammalia, Penn Monthlj'^, December, 1875. 
t Amer. Journ. Sci. and Arts, May, 1873. 
. I Homologies and Origin of the Molar Teeth in Mammalia Educabilia, March, 
1874, p. 14. 
N 
