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Show 1 9 0 5 .] BRAIN OF LIZARDS. 2 6 9 by them. It is faintly grooved in the middle line and laterally on each side is a flattened process extending backward rather beyond the rest of the cerebellum. Its insignificant proportions are shown by the fact that the transverse (antero-posterior) diameter of this thin plate is 2 mm., while the corresponding measurement of the optic lobe is 8 min. As will be seen from text-figs. 39, 40 (p. 268), the cerebellum of Varanus exanthematicus is a much more important structure. Not only the actual but the relative size of the cerebellum is greater. The corresponding measurements to those given above for Tupinambis are for Varanus-diameter of cerebellum 4*5 mm., diameter of optic lobes 4*5 mm. They are thus equal. The difference in dimensions between the cerebella of the two Lacertilia is due to the exaggeration in Varanus of the boss-like eminence upon the cerebellum of Tupinambis and Iguana. Not only is the cei'ebellum of Varanus exanthematicus much greater in bulk than that of Tupinambis or Iguana, but it is more complicated in structure owing to furrows. The dorsal furrow, continuous with that dividing from each other the corpora bigemina, is more deeply marked in Varanus and more definitely circumscribed than in Tupinambis ; in Iguana 1 did not find any traces of it. In the second place, the cerebellum of Varanus exanthematicus has an equally deeply marked lateral furrow, which runs obliquely upwards and forwards. Thirdly, the lateral process of the cerebellum is much more sharply marked off from the cerebellum itself than in Tupinambis, and runs downwards rather than backwards, thus distinctly suggesting the flocculus in the cerebellum of the higher forms. It is, indeed, not at all unlike the cerebellar flocculus in Alligator. It is plain therefore that the cerebellum of this Lizard is not " a mere transverse plate," but an organ of some dimensions, and, indeed, not very far, in point of relative size, from that of the Crocodilia. A large cerebellum has been associated in reptiles with the swimming habit. And it is true that the Monitor Lizards are often largely aquatic in habit. Curiously enough, however, the present species, with its large cerebellum, is stated by Dr. Gunther* not to take to the water. More likely, as it appears to me, is this advance in structural complexity of the brain to be associated with the not only isolated but high position which the Monitors occupy among the Lacertilia. (2) On the Cerebral Hemispheres in Tropidurus hispidus. I imagine that I am right in believing that the brain of this Iguanoid Lizard has not up to the present been submitted to anatomical examination. I am able, therefore, to add a fact of * " On the Anatomy of Regenia ocellata," P. Z. S. 1861, p. 60. |
