PERISSODACTYLA. 
259 
described by Leidy under the name of Hipposyus strongly resemble those 
of Syracotlierium, but belong, as I now believe, to another genus. The 
inner cones are always distinct in Hyracotlierium^ but confluent in Hipposyus; 
the latter also lacks the posterior median tubercle. 
Two species of Hyracotherium have been described by Owen from the 
London clay, H. leporinum and H. cimiculus, and a third has been discov¬ 
ered in the siderolitic beds of Yaud, the II. sideroUthicum of Pictet. The 
species of both continents differ among themselves in the relative degree 
of development of the median tubercles of the superior molars, being in 
some almost conical, in others subtransverse. The former character is 
seen in the H. cnspidatimi, Cope, and, according to Professor Owen’s figure, 
in the H. leporinum. Every intermediate form can be observed. 
The affinities of this genus have been variously interpreted, but its 
original reference to the Perissodactyla by Owen is rendered certain by the 
discovery of the structure of the astragalus in the R. tapirinim, Cope, and 
of the limbs in the closely-allied genus Orotlierium (see page 253). If, as I 
suspect, the OroMp)pus of Marsh is identical with Hyracotherium, the struct¬ 
ure of the feet described by that writer as belonging to it is also conclusive 
evidence to the same effect. 
As regards the closer affinities of Ilyracotherium, Professor Owen re¬ 
marks (Paleontology, p. 329, 1860), in describing the nearly-allied genus 
Pliolophus: ^^Pliolophus and Hyracotherium form a well-marked section in the 
Lophiodont family, which seems to have preceded the Palseotherian family 
in the order of appearance, and to have retained more of the general Ungu¬ 
late type than that family. This is shown by the graduation of the Tapiroid 
modification of the molar teeth into one more nearly resembling that of the 
Anthracotheria and Choeropotami; by the absence of the postero-internal cone 
on the ultimate premolar, by which all the premolars are, as in Artiodactyles, 
less complex than the true molars; by the form and position of the nasal 
bones; and by the structure of the external nostril” Professor Owen ex¬ 
presses the opinion that Pliolophus has three digits on the hind foot. 
In a paper “On the Primitive Types of the Orders of the Mammalia 
Educabilia”,* the writer pointed out (p. 7) that the type of the inferior molar 
* Proc. Amer. Philos. Soc., extras, May 6, 1873. 
