PAL^OSYOPS LEIDY, AND ITS ALLIES. 
309 
heavy ; its tuberosity is Ijroad, bevelled oft' and shows no subdivision like that of 
the Rhinoceros and Tapir. Tlie superior external border of the tibia is not incised 
as in the Tapir for the transmission of the tendon of the anterior extensor. This 
notch is only slightly marked in Rhinoceros. In the Equine series the notch 
for the extensor tendon is very strongly marked, especially so in the recent Horse. 
The longitudinal fossa? of the upper portion of the shaft are sti'ongly developed and 
deep. The middle section of the shaft is flattened, thus reducing the diameter of 
the internal border of the bone. The distal extremity of the tibia is broad and 
flattened. The trochlea? ai'e shallow, the external border of the outer being deeply 
excavated for the fibula. The posterior trochlear tuberosity is short and not promi- 
nent. The superior contact of the fibula and tibia forms quite a deep depression, 
and the tibia shows a distinct flat facet for articulation with the fibula. In the 
Rhinoceros this facet for the fibula is absent. 
Tarsus. (PI. XIV, figs. 46-49). — There is an abundance of material in the 
collection pertaining to the pes of this species, and, moreover, an astragalus 
and a calcaneum are associated with the posterior extremity already referred 
to, so there is no doubt as to the correct identification of the tarsus. The figure 
of the foot is a composition, the tarsal elements, other than the astragalus and 
calcaneum, not having been found with the extremity aliove mentioned. The foot 
of P. pahidosus as compared with that of P. 7ninor and Liimiohyops is very much 
heavier, being broader and longer. The form and arrangement of the tarsal’ facets 
are quite different from those of the related species and approach more nearly the 
conditi(?n found in Diplacodon, although in the latter genus some of the facets 
are the reverse in size of those of P. pahidosus. Compared with the recent Per- 
issodactyles, such as the Tapir, we notice a great change in the size and arrange- 
ment of the tarsal elements, and in order to make the description clearer, 1 shall 
enumerate some of the most important variations in the tarsus of this species as 
compared with that of Tapirus. The most striking difterence between the two 
tarsi 2 )ertain to the ectocuneifbrm and its facets. In P. pahidosus tins bone is very 
high and narrow and the jiroximal portion of metatarsal IV is likewise narrowed, 
conse(|uently there is no articulation between the latter and the ectocuneifonn ; 
in other words, the reverse type of tarso-rnetatarsal articulation does riot occur in 
this species as it does in Tapirus indicus. The cuboid moreover, has a slight con- 
tact with metatarsal III. The astragalo-cuboid facet is very large. The articular 
faces of the tarsal bones are flatter than in the tarsus of the Tapir. And lastly 
there is not so decided a difference in size between metatarsal III and the lateral 
metatarsals as in the Tapir. 
Calcaneiwt. — The calcaneum in this species is short and broad. Its articular 
portion is particularly massive. The tuber is short, the distal part of the same 
being club-shaped and very rugose. The neck of the calcaneum is slightly com- 
pressed, with a considerable depth. All the fiicets of the calcaneum are widely 
separated. The ectal facet is very large, convex, and sends a narrow tongue-shaped 
