1905.] BRAIN OF LIZARDS. 269 



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 

 measiu'ement of the optic lobe is 8 mm. 



As will be seen from text-figs. 39, 40 (p. 268), the cerebellum 

 of Varanas exanthematicus is a much more important structure. 

 ISTot only the actual but the relative size of the cerebellum is 

 greater. The corresponding measurements to those given above 

 for Ttqnnambis are for Varauus — 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 Va7'anus of the boss-like 

 eminence upon the cerebellum of Tupinambis and Iguana. ISTot 

 only is the cerebellum of Varanus exanthematicus much greater 

 in bulk than that of Tiqnnamhis 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 I 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 

 latei'al process of the cerebellum is much more sharply marked 

 ofl:' from the cerebellum itself than in Tupinmnbis, 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 aqviatic in habit. Curiously enough, however, 

 the present species, with its large cerebellum, is stated by 

 Dr. Giinther* 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. 



