14 PB0FESS0E OWEN ON THE CHABACTEBS, ETC. 



The brain is that part of the organization which, by its superior 

 development, distinguishes the Mammalia from all the inferior 

 classes of Vebtebbata ; and it is that organ which I now propose 

 to show to be the one that by its modifications marks the best and 

 most natural primary divisions of the class. 



In some mammals the cerebral hemispheres are but feebly and 

 partially connected together by the ' fornix ' and ' anterior commis- 

 sure : ' in the rest of the class a part called ' corpus callosum ' is 

 added, which completes the connecting or 'commissural ' apparatus. 



With the absence of this great superadded commissure* is asso- 

 ciated a remarkable modification of the mode of development of 

 the offspring, which involves many other modifications ; amongst 

 which are the presence of the bones called ' marsupial,' and the 

 non-development of the deciduous body concerned in the nourish- 

 ment of the progeny before birth, called ' placenta ; ' the young in 

 all this ' implacental ' division beiug brought forth prematurely, 

 as compared with the rest of the class. 



This first and lowest primary group, or subclass, of Mammalia 

 maybe termed, from its cerebral character, Lyencephalaj-, — signi- 

 fying the comparatively loose or disconnected state of the cerebral 

 hemispheres. The size of these hemispheres (fig. 1, a) is such that 

 they leave exposed the olfactory ganglions (a), the cerebellum (c), 

 and more or less of the optic lobes (b) ; their surface is generally 

 smooth ; the anfractuosities, when present, are few and simple. 



The next well-marked stage in the development of the brain is 

 where the corpus callosum (indicated in fig. 2, by the dotted lines 

 d, d) is present, but connects cerebral hemispheres as little ad- 

 vanced in bulk or outward character as in the preceding subclass ; 

 the cerebrum (a) leaving both the olfactory lobes (a) and cerebel- 

 lum (c) exposed, and being commonly smooth, or with few and 

 simple convolutions in a very small proportion, composed of the 

 largest members of the group. The mammals so characterized 

 constitute the subclass Lissencephala* (fig. 2). 



In this subclass the testes are either permanently or temporarily 

 concealed in the abdomen : there is a common external genito- 

 urinary aperture in most ; two precaval veins (' superior ' or 

 ' anterior venae cava? ') terminate in the right auricle. The squa- 

 mosal in most, and the tympanic in many, retain their primitive 

 separation as distinct bones. The orbits have not an entire rim 



* " On the Structure of the Brain in Marsupial Animals," Philos. Trans. 

 1837, p. 87. 

 f Xuw, to loose ; eyiceQaXos, brain. J \«r<ros, smooth ; tyice^oXos, brain. 



