ENDOCRANIAL ANATOMY OF FOSSIL MAMMALS 355 
The ectosylvian sulcus (S.ectosyl.), absent or only faintly 
indicated in Dinictis (fig. 4), is represented in the present cast 
by a shallow semicircular groove. It is more clearly indicated 
on the left than on the right side, although its course is clear in 
either case. It begins and ends without joining either of the 
adjacent sulci. The depression labeled sylvian fossa (F’.syl.) is 
much narrower and more clearly impressed than in Dinictis. It 
is not, however, so advanced as in Smilodon (fig. 16), and Smilo- 
don is not so far advanced in this respect as the modern lion. 
So that we have four stages of development of the sylvian fissure 
represented. Dinictis represents the most primitive (fig. 4). 
Here the fossa is wide and its floor smooth. The undetermined 
felid (Hoplophoneus?) (fig. 10) represents the next stage of 
advance followed in sequence by the Pleistocene machairodont, 
Smilodon (fig. 16), and culminating in the lion. The first three 
forms are doubtless in direct ancestral sequence, but I do not 
mean to say that the modern lion is derived from the Pleistocene 
machairodonts. Anteriorly and posteriorly the sylvian depres- 
sion, in the undetermined felid (fig. 10), is joined by shallow 
depressions which may represent sulci, the anterior one repre- 
senting the rhinal fissure, and a faint fissura rhinalis posterior 
indicating an extensive expansion of the neopallium. 
The base of the brain is clearly preserved (fig. 11) and indi- 
cates a variety of structures. The olfactory region is broken 
away (fig. 11, X). The optic nerves (N.opt., fig. 11) are indi- 
cated in the bases of the casts of the optic foramina. The 
optic chiasma is represented as a narrow structure with evidences 
of a median sulcus. The chiasma is rather longer than in the 
modern cat. The optic tracts are visible as smooth elevations 
on either side of the anterior portion of the pituitary body (fos. 
pit.), which is, of course, represented by the casts of the cavity 
of the sella turcica. The pituitary is longer and more oval in 
this ancient cat than in the modern form. Representing as they 
do diverse lines of the Aeleuroidea, we should not expect a close 
conformity of endocranial structure. 
Lying lateral and somewhat posterior to the hypophyseal 
elevation are three pairs of structures which I have interpreted 
