NERVOUS SYSTEM. VERTEBRATA. 195 



D. 236. A cast of the cranial cavity of a Oolugo (Galeopithecus 

 volans). [In duplicate.] 



This cast shows the true shape and proportions of the 

 olfactory bulbs, cerebral hemispheres, and the main mass 

 of the cerebellum. It gives some idea as to the exact 

 area of the quadrigeminal bodies exposed. 



The rhinal and "modified suprasylvian " furrows are 

 well shown. 



Gervais, Journ. de Zool., t. i. 1872, p. 445. 



ORDER RODENTIA. 



Section SCIUROMORPHA. 

 Family CASTORIDJE. 



D. 237. The brain of a European Beaver (Castor fiber). 



Most parts of the brain of the Rodents present features 

 such as are common to most of the Eutheria ; but in 

 respect to the arrangement of the sulci of the cerebral 

 hemispheres, the Rodentia are the most aberrant group of 

 the Eutheria (excepting perhaps the peculiarly-modified 

 Sirenia). 



The most noteworthy features in the brain of the 

 Beaver are the obliteration of a great part of the rhinal 

 fissure and the utter absence of sulci in the large pallium. 

 The meaning of this is not altogether clear, but appearances 

 suggest that the growth of the brain in a roomy cranial 

 cavity which has exercised little restraint on the growing 

 hemispheres may explain this absence of sulci and 

 imperfection of the rhinal fissure. But even admitting 

 this, the smoothness of such large hemispheres is a peculiar 

 phenomenon, which is much rarer than is generally supposed. 

 Thus even in the small Erinaceus, among the primitive 

 Insectivores, there is a well-developed rhinal fissure and 

 distinct traces of at least one neopallial sulcus ; and in 

 Gymnura and, especially, Galeopithecus there are deep sulci, 

 in spite o the fact that these brains are much smaller than 

 that of the Beaver and represent a more primitive type. 

 Then, again, among the Hystricomorphine Rodents we find 



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