PERISSODAOTYLA, 
323 
have shown to be more nearly allied to Ilippotherium than to Equus. In 
1873, I published the first account of the structure of many parts of the 
skeleton, including the limbs, showing that the genus is three-toed. Some 
time previousl}^,* * Professor Leidy had asserted that the genus Hippidium of 
Owen is identical with Protoliippus of earlier name, and he refers the South 
American species, on which Professor Owen based his genus, to Protoliippus. 
In 1875, Professor Burmeister published a full description f of the osteology of 
the Hippidium neogaum., Lund, from the post-Tertiary deposits of Buenos 
Ayres, showing that that animal possesses but a single digit on all the feet 
It is, therefore, quite distinct from Protoliippus., and constitutes an important 
link in the series of Equine genera,— i. e., the one between Hquus and 
Protoliippus. Certain fossil Horses from the Loup Fork beds of the Niobrara 
River present, according to Marsh, the same characters; but he names 
them Plioliippus,X being apparently unaware of the prior applicability of 
Owen’s name. Slight differences between the North and South American 
species in the concavity of the face and form of the nasal bones are prob¬ 
ably specific only. 
Antecedent to Protoliijjpus in its dental characters comes the genus 
Hippotlierium, which has been for some years recognized by authors as con¬ 
necting Hquus with the Miocene Ancliitlierium. Palaotlierium, Cuv., had 
been indicated by Huxley as the probable Eocene ancestor of Ancliitlierium, 
but I have shown that it was much more probably Hyracotlierium. This 
view has been confirmed by Marsh, who treats of this genus as I have iden¬ 
tified it in America, under the.synonym Oroliippus (see supra, page 260). 
Protohippus, spec, inclet. 
Plate Ixxv, fig. 7. 
A single much worn molar, of the size of that of P. sejunctus, apparently 
represents this genus. 
The specimens obtained indicate another species of Horse, but they are 
not sufficiently characteristic for determination. 
» ■ ■■ - • - - • — . - .. . ■■■ ■ ■■ 
* Proc. Acad. Phila., 1870-1. 
t Los Caballos Fossiles de la Pampa Argentina, 1875, pp. 37-43. 
f Amer. Journ. Sci. Arts, 1874, p. 252. 
