PAL^OSYOPS LEIDY, AND ITS ALLIES. 
327 
77iegarhmus. In the latter the vertical height of the cerebral lossa is no greater 
than that of the cerebellar fossa, whereas in recent Perissodactyles, where the cere- 
bral lobes are very large and have a great vertical extent, tlie cereljral fossa ranges 
high above the cavity for the cerebellum. The anterior limit of the cranial cavity 
in P. 7negarhinus is on a vertical line with the middle of the alisphenoid canal. 
In the Tapir this cavity is prolonged farther forward and reaches about the 
middle of the wings of the sphenoid. Owing to the low reptilian-like form of brain 
that occurs in this species the arrangement of the subdivisions of the cranial cavity 
follow one another in regular order, and there is not that superior position of the 
olfactory fossse that is seen in recent fonns. In other wmi’ds the floor of the cerebral 
cavity is nearly straight, and this is especially noticeable in the region anterior to 
the pituitary fossa, which is not so oblique and ascending as in the Tapir’s skull. 
Another primitive character of the cranial cavity of this species is that the cere- 
bellar fossa is strongly marked off from the cerebral by a well developed ossified 
tentorium which extends all around the walls and roof of the cranial cavity, 
and is as strongly marked in the skull of P. megarhhms as it is in that of the Carni- 
vora. In the skull of the Bear, for example, the large size of the tentorium is notice- 
able. In recent Perissodactyles the pi-esence of an ossified septum between the two 
posterior subdivisions of the cranial cavity is reduced to a minimum ; in the Tapir 
it is totally wanting ; in some species of Rhmocei'os it is also absent, whereas in 
other species of this genus a single elongated curtain-like process may be developed 
upon the roof of the cranial cavity. This is also the case in Eqims. The olfactory 
fossm are elongated and well separated from the cerebral cavity. The elongation 
of these fossm is a noticeable character of this species. The olfactory fossa of each 
side is separated from that of the other by a stout septum, which rises from the 
anterior floor of the cerebral cavit}’. In recent forms the olfactory cavities are carried 
upward and are oblique in position, but their primitive position is found in Palceosy- 
ops. In correlation with the great lateral reduction of the brain of this species is 
the encroachment of the brain-case ujion the encephalic mass. The walls of the 
brain-case are extremely thick and heavy; this is especially noticeable in the roof 
of the skull, which is over twice as thick vertically as in the Tapir’s skull. The 
reduction of the brain is especially appreciated when the extremely deep temporal 
fossm ai’e seen from without. 
B7'ai7i, (PI. II, figs. 6, 7.) — This is the only species of the subfamily whose brain 
we are able to figure. There are a number of brain-casts in the collection, all being 
from the same skull, No. 10,041. The brain of Palceosoyps megm'hinus, like most of 
the Eocene Ungulates, is very small, and when we compare the size of the brain in 
the Tapir with that of this species we are at once struck by the great discrepancy 
in their size. In the form and arrangement of its lobes the brain of P. 77iegarh77ius 
is intermediate between those of Phc7iacodiis and Taph'ics. Compared with the 
size of the skull the brain of P. 77iega7'hm7is was larger than that of Titanotfm'hiiTt. 
In the latter genus the breadth of the encephalon is unusually developed as compared 
