196 PHYSIOLOGICAL SERIES. 



cerebral hemispheres of the same dimensions as, or even 

 smaller than, the Sciuromorphine Castor, possessing numerous 

 deep sulci. 



This is one of the enigmas of cerebral morphology 

 which we are utterly unable to satisfactorily explain at 

 present. (). C. 1323s. 



Beddard, Proc. Zool. Soc. 1892, p. 597. 



D. 238. The brain of a Beaver (Castor Jiber), dissected to show 

 the distribution of the fibres of the corpus callosum. 



This dissection was made by Prof. Richard Owen for 

 purposes of comparison with the brain of the Wombat 

 (D. 224). Its object was to demonstrate by comparison 

 the absence of the corpus callosum in the Marsupial. 



R. Owen, Phil. Trans. 1837, p. 89. 0. C. 1323 B a. 



D. 239. The brain of a Beaver (Castor fiber), dissected to show 

 the hippocampus and fornix. 



This dissection was also made by the late Prof. Richard 

 Owen. By comparison with D. 238, it will be seen that 

 when the corpus callosum has been removed, as in this 

 specimen, an arrangement of the fornix is exposed which 

 is analogous to that which may be seen in the Marsupial 

 (D. 224) without removing any such commissure. Hence, 

 Owen argued, the corpus callosum is lacking in the 

 Marsupial. 



This rough dissection does not clearly demonstrate the 

 further fact that in the Rodent the hippocampus does not 

 extend forward so far as it does in the Marsupial. 



O.C. 1323 Eb. 



D. 240. The brain of an American Beaver (Castor canadensis) , 



There is a shallow sagittal furrow on the dorsal surface 

 of each hemisphere, which might ivpivsi-nt. < -itlirr tin- 

 oorono-latenJ or the suprasylviao sulcus. The latter is the 

 more stable and precocious of the two sulci, and is rarclv 

 absent when the lateral sulcus -xi>ts. Yet we kno\v lor 

 certain that this sometimes happens in the case of 

 Tamanduas (Edentata). 





