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Show 1905.] ARTERIAL SYSTEM IN SAUROPSIDA. 61 § Brain q/' Tropidurus hispidus. In comparing the arterial system of the brain of this Iguanoid with those of the other species of Lacertilia with which 1 have dealt, I am unable to say anything about the cerebellar arteries, which were not visible in the specimen examined by me. The bifurcation of the 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 corpora 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 Lacertilia. § Brain of Eumeces algeriensis. The arrangement of the arteries of the brain in this Skink, which, so far as I am aware, has not been described, shows certain differences 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 with the basilar 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 anteriorly begins further back than in both I aranus 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 arteries join the circle of Willis well in front 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 which runs parallel 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 anterior cerebral, which gives off the ophthalmic artery, is considerably thicker than the middle cerebral artery. § Brain of Gerrhosaurus. As is the case with Eumeces, the basilar artery in Gerrhosaurus (see text-fig. 17, p. 62) divides rather further back than it does in either Iguana or Varanus. There is, moreover, a very distinct inequality of calibre in the two arteries ; the right is in fact considerably 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 origin of the anterior |