x JouRNAL OF COMPARATIVE NEUROLOGY. 
sylvian sulcus is always separate from the pseudosylvian, and is gener- 
ally joined to the postsylvian sulcus. In the Family Lemuride the 
suprasylvian sulcus is always (or practically always) separated from the 
parallel (postsylvian) sulcus, but numerous fragmentary sulci, and a 
backwardly-directed hook at the upper end of the suprasylvian (Syl- 
vian) sulcus, or a forwardly-directed hook to the postsylvian (parallel) 
sulcus, serve to remind us of the old link between these two sulci, 
which has been broken. 
The lower end of the suprasylvian sulcus in the Lemurs overlaps 
the upper part of a pseudosylvian sulcus (of the feline type), the gyrus 
between the two sulci becomes submerged, and the resulting sulcus we 
- now call the ‘‘Sylvian fissure.” The lower end of the suprasylvian 
sulcus can be seen in many Prosimian hemispheres emerging from 
the front of the ‘‘Sylvian complex” a short distance above the rhinal 
fissure. 
In the Apes the submerged area increases in extent and is called 
the ‘‘insula.’ It is hidden by two opercula; and a comparison of a 
large series of Ape-brains seems to clearly demonstrate that the dorsal 
limiting sulcus of the insula is no other than the suprasylvian sulcus. 
In no brain does this sulcus extend so far (in the ventral direction) 
as the rhinal fissure. In many of the larger Apes it emerges slightly 
and cuts into the anterior lip of the Sylvian fissure. In Aylobates, 
Simia, and the Anthropopithect it extends forwards upon the surface so 
as almost to reach the fronto-orbital sulcus. 
The early history of the latter sulcus is not satisfactorily known. 
It is present in an exceedingly well-developed condition in all the 
Simiidz, and in a less obtrusive form in many of the larger Apes; but, 
on the other hand, it is absent in many of the Cebidz and Cercophthe- 
cide. Such being the case, it is very surprising to sometimes find in 
the Lemurs a small sulcus, which can be no other than the fronto- 
orbital. It is impossible to say with any degree of probability whether 
this sulcus is represented beyond the limits of the Primates. The di- 
agonal sulcus of the Carnivora, Ungulata, Edentata (Bradypus, Myrme- 
cophaga) occupies a position analogous to that of the fronto-orbital in 
the Primates. 
In the Anthropoid Apes there is a pronounced tendency for the 
anterior lip of this (fronto-orbital) sulcus to become opercular and to 
extend backward over the insula, the anterior limit of which is marked 
out by the sulcus itself. 
This process of operculation may be carried very far even in 
Hylobates, Simia, and Anthropopithecus troglodytes ; and in one specimen 
of Anthropopithecus gorilla (D. 656) a very close though spurious imita- 
tion of the human condition of this region is attained. 
In the human brain this process of operculation generally leads to 
‘the complete covering of the insula. The anterior lip of the fronto- 
orbital (or, as we may now call it, anterior limiting sulcus of Reil). 
grows backward to meet the temporal operculum, and thus gives rise to 
the ‘‘stem” of the Sylvian fissure. The dorsal lip of the forward ex- 
tension of the superior limiting sulcus grows down to meet the temporal 
