1905. ] ARTERIAL SYSTEM IN SAUROPSIDA. 69 
olfactory branch arises beneath the olfactory lobe at the junction 
of the latter with the cerebral hemisphere ; before this point of 
junction a branch is given off to the olfactory lobe. 
The circle of Willis is completed anteriorly ; it also ends in two 
strong branches which run along the under surface of the brain, 
anteriorly, giving off numerous branches at the junction of the 
hemispheres with the olfactory lobes. One or more of these bend 
downwards (as the brain is viewed from beneath) and pass through 
the gap between the hemispheres running to the dorsal side of the 
brain in a way which suggests the callosal artery of the mammals. 
I could not detect anything more than a very small branch arising 
where the ophthalmic arteries arise in the Lacertilia. I cannot 
think that this artery is absent, but it is clearly not so conspicuous 
as in the Lizards. 
It is evident that the encephalic arterial system differs quite as 
much from that of either Lizards or Snakes as do the encephalic 
arterial systems in the two last mentioned groups. 
It is thus plainly possible to distinguish between several types 
of distribution of the cerebral arteries among the different divisions 
of the Sauropsida, and there is, as is well known, another type 
characteristic of mammals. It will be interesting to ascertain 
how far these several types confirm views as to the relative 
positions of the groups of Sauropsida under consideration. It will 
not be held by anyone, I presume, that the class Aves represents 
a primitive Sauropsidan type; and in agreement with this 
presumption we find clear evidence of modification in the 
encephalic arteries*, in the abortion of one or other of the 
normal two branches of the basilar. On the other hand, the 
arteries in question of birds are, as I think, undoubtedly primitive 
in that there is no completion of the circle of Willis anteriorly. 
A completed circle of Willis appears to me to be a secondary 
modification mainly for the reason that in mammals, where it 
occurs universally, it is there brought about in more than one way, 
and is moreover associated with strong arteries in the anterior 
region of the brain in close communication, or rather in close 
apposition, and there is apt to be confluence between closely 
apposed spaces and vessels. If this view be correct, we can set 
aside the brain of the Python and that of Vestuwdo as showing 
primitive characters by virtue of the fact that they have a closed 
circle of Willis. And in addition to this, it may be pointed out 
that the asymmetry of the arterial system in the Snake indicated 
by the carotids, and the changes in the disposition of the vessels 
due to the prevalence of the left carotid, can be fairly regarded as 
being secondary. This conclusion is obviously in accord with the 
current views of the relations of the Ophidia to other reptiles. 
There only remains the Lacertilia. 
It is, in my opinion, probable that in this group (and in 
Hatteria) the whole question lies of the antiquity of existing 
* P.Z.S. 1905, vol. i. p. 102. 
