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Show 270 ON THE BRAIN OF LIZARDS. [June 6, some little interest to what is already known about the Lacertilian brain, as a result of the examination of two brains of this Lizard. In the Lacertilian brain generally, so far as my own knowledge and the inspection of published figures * enable me to state, the optic lobes lie behind the cerebral hemispheres, the furrow between them being practically vertical; there is, in fact, no trace of an overlap of the corpora bigemina by the hemispheres. In the Chelonia, on the other hand, it has been recognised that some forms show an overlap of the corpora bigemina by the cerebral hemispheres. I have found this lobe very obvious in a brain of the large Testudo vicina, the vascular system of which I have recently described f. The overlap, however, is lateral and not dorsal. It is quite different with Tropidurus. There is a very distinct overlap of the corpora bigemina by the hemispheres dorsally. The corpora bigemina are thus partly hidden when the entire brain is viewed on the dorsal aspect. A comparison of the measurements of the brain in this species and in Iguana tuberculata seems to throw some light upon the causation of this overgrowth of the cerebral hemispheres over the corpora bigemina dorsally. The following are the measurements to which I desire to refer:- Iguana. Tropidurus. mm. mm. Length of brain to end of cerebellum ... 16 11 Length of cerebral hemispheres ........... 9 6 Breadth of cerebral hemispheres ........... 11*5 6‘5 It will be observed, from a comparison of these figures, that the proportions between the total length of the brain in the two Lizards, and both the breadth and length of the cerebral hemispheres, are about equal. It therefore results that the overlap of the hemispheres in Tropidurus is rendered necessary by the skull formation and consequent lack of room for increased lateral growth of the hemispheres. By growing over the corpora bigemina, the hemispheres have been able to attain to the proper size necessary to the equilibrium of their possessor. These considerations may be regarded, perhaps, as discounting the morphological importance of the partial covering over of the corpora bigemina by an extension backwards of the cerebral hemispheres. Nevertheless, it is impossible to overlook the fact that there is an approximation in the brain of this Lizard, to whatever cause it may be due, to those of higher Vertebrates. * See Bronn's Klassen u. Ordnungeu des Thierreichs, Bd. vi., and Meyer, Zeitschr. wiss. Zool., Bd. Iv. (1893). f Supra, p. 67. |