328 
A MEMOIR UPON THE GENUS 
with its length The axes of the fore and hind brain are continnons, and are not 
bent npon each other as in the Tapir’s brain, and that of the higher Ungulates 
Ceredrum— The prosencephalic lobes are very broad, short and somewhat 
depressed ■ their general outline is a broad oval with only a slight distinction between 
the frontal and occipital portion. Viewed from the side the temporal lobe is much 
less marked than in the Tapir’s brain. The inferior surface of the frontal portion 
of the cerebrum is convex and not concave as in the Tapir. The surface of the 
hemispheres is well convoluted ; and in this respect this species shows a marked 
advance over such early Eocene forms as Phenacodtis and Coryphodon. The .Sylvian 
sulcus is well developed and vertical in position. There are also well marked pre- 
and poshsylvian sulci, in continuity with the main sulcus. The surface of the brain 
between the Sylvian sulcus and the hippocampal is smooth and strongly convex ; as 
there is no fissure between the two last mentioned sulci we may conclude that the 
Rhinal and Sylvian sulci have coalesced as in the Carnivora. In the Tapir and 
Rhinoceros the .Sylvian and Rhinal sulci are distinct, and the presylvian is separ- 
ated from the Syiviaii proper, although its primitive condition is in connection with 
the latter. A long wavy fissure above the superior termination ol the .Sylvian sulcus 
is evidently the suprasylvian fissure. It extends from the posterior part ot the 
occipital lobe well forward to the middle portion of the frontal; in its couise it 
throws off’ a number of secondary sulci. There is an indication that the crucial 
sulcus was present in this brain, but the coronal, so characteristic of the brains ol 
recent Ungulates, is absent. The lateral portion of the occipital lobe above the 
posterior prolongation of the Sylvian sulcus is provided with an o))lique fissure, 
which is probably the one called by Krueg the posterior fissure. The convolu- 
tions of the brain in P. megarhinus compared with those of the Tapir are less numer- 
ous and complex. In the Tapir’s brain the longitudinal secondary (issures are more 
numerous, and the frontal lobes of the hemispheres ai'e much larger than in P. 
megarhinus. This region is also more convolute in the Tapir. The olfacfi)ry lobes 
in the brain of P. megarhimis are large and strongly prolonged anteriorly. They 
differ in form from those of recent Perissodactyles, where they are more vertically 
placed, their long axes being from above downward and closely connected along 
their whole extent with the surface of the hemispheres. The olfactory lo)>es in P. 
megarhinus were separated, and probably ninch longer than represented in the 
drawing, as in the cast they appear to Ije. abruptly cut off’. 
Midhrain. — In Phenacodus primcevus the prosencephalic lobes arc widely sepa- 
rated from the cerebellum, leaving the midbrain region exposed as in reptiles. In 
Paleeosyops megarhinus the brain is more highly developed than in Phenacodus 
primcevus. Although the cerebral lobes do not reach the hind brain, they were 
prolonged probably far enough behind to cover the corjjora quadrigemina. 
Cerebellmn. — Tlie hind brain is imusually large and broad in this species. The 
cerebellum is as wide transversely as the whole extent of the prosencephalic lobes ; 
it is subdivided into three narrow lobes, the median being the largest and most con- 
