1905.] ARTERIAL SYSTEM IN' SAUROPSIDA. 61 



§ Brain q/" Tropidurus hispicUis. 



In comparing the arterial system of the brain of this Iguanoid 

 with those of the other species of Lacertilia with which I have 

 dealt, I am unable to sa}^ anything about the cerebellai- arteries, 

 which were not visible in the specimen examined by me. The 

 bifurcation of tlie basilar artery in front at rather an acute angle 

 consisted of equally-sized vessels, and the carotids which joined 

 these arteries behind the third pair of nerves were also equal. 

 The arteries to the corpoi-a bigemina disappear at once in the 

 groove separating each corpus bigeminum from the hind brain. 

 The other arteries of the brain seem to be as in other Lacei'tib'a. 



§ Brain o/Eumeces algeriensis. 



The ari-angement of the arteries of the brain in this Skink, 

 which, so far as I am aware, has not been described, shows 

 certain difi'erences from that of both Varanus and Iguana. 

 These features are illustrated in the accompanying drawing 

 (text-fig. 16, p. 62). The fusion of the vertebral arteries Avith 

 the basilai' marks, as is usual, the end of the medulla. From the 

 basilar artery arise a number of branches of which the posterior 

 cerebellar arteries are the most important ; of these the left artery 

 arises in advance of the right and it is shortly reinforced by 

 another branch. The bifurcation of the basilar anterioily begins 

 further back than in both Varanus and Iguana ; and another 

 difference from the conditions observable in these two genera is 

 to be noted. In these Saurians the carotids join the circle of 

 Willis behind the origin of the third pair of nerves ; in Eumeces 

 these artei'ies join the circle of Willis well in fi-ont of the third 

 nerves, and therefore also in front of the slender anterior 

 cerebellar arteries, and of the artery supplying the corpus 

 bigeminum on each side. This artery not only supplies the 

 corpus bigeminum but also the cerebellum, and it sends a branch 

 forward wdiich runs pai-allel to the posterior cerebral artery, and 

 like it is lost in the groove separating the fore brain from the 

 mid brain. Between this artery and the middle cerebral or 

 Sylvian is a slender twig like that of Iguana which runs to the 

 base of the optic nerves. The anteiior cerebral, which gives off 

 the ophthalmic artery, is considerably thicker than the middle 

 cerebral artery. 



§ Brain o/Tierrhosaurus. 



As is the case with Eumeces, the basilar artery in Qerrhosaurus 

 (see text- fig. 17, p. 62) divides rather further back than it does in 

 either Iguana or Varanus. There is, moreover, a very distinct in- 

 equality of calibre in the two arteries ; the right is in fact considei-- 

 ably larger than the left. This inequality does not, however, extend 

 to the two carotids, which are equal in size. These join the circle 

 of Willis only just in front of the point of oiigin of the anterior 



