70 INTRODUCTION. 



brain, in the various Marsupial animals, and the result 

 of this investigation was a most interesting discoverv. 

 Besides the decreased size of the hemispheres of the 

 brain, and consequent exposure of the cerebellum, 

 indicative of a low grade of organization, the corpus 

 callosum and septum lucidum were found to be 

 entirely wanting in these animals, or at least, exist- 

 ing only in a rudimentary state. Now the corpus 

 callosum, which is the principal bond of union 

 between the opposite hemispheres of the brain, had 

 been regarded as the great characteristic of the brain 

 in the Mammalia, and in fact this commissural 

 apparatus presents the essential difference which 

 exists between that, and the oviparous vertebrate 

 classes. 



'' The agreement of the Marsupial animals," says 

 Professor Owen, " in so important a modification of 

 the cerebral organ as the absence of a corpus cal- 

 losum and septum lucidum, affords additional and 

 strong grounds for regarding them as a distinct and 

 peculiar group of Mammalia ; and when to this 

 modification of cerebral structure are added the 

 traces of the oviparous type of structure, presented 

 in the circulating and absorbent systems, together 

 with the peculiarities of the osseous and generative 

 apparatus, we may with reason suspect that distri- 

 bution of the MarsKpiata to be artificial, and founded 

 on an imperfect knowledge of their mutual affinities, 

 which, from a modification of the teeth and extremi- 

 ties alone, would separate and disperse the species 



