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 ofi" to the olfactory lobe. 



The circle of Willis is completed anteriorly ; it also ends in two 

 strong branches Avhich 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 

 downw^ards (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 anj^hing 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 raammals. 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 comjjletion of the circle of Willis anteriorly. 

 A completed chicle of Willis appears to me to be a secondary 

 modification mainly for the reason that in mammals, where it 

 occurs universally, it is there bi-ought 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 Testudo as showing- 

 primitive characters by virtue of the fact that they have a closed 

 circle of W^illis. And in addition to this, it may be pointed out 

 that the asjonmetry 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 cai-otid, can be faii4y 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. 



