52 Classification of the Mammalia. 



division of the mammalian class, based upon the four leading 

 modifications of cerebral structure in that class. 



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

 development, distinguishes the Mammalia from all the inferior 

 Masses of Vertebrata ; 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 com- 

 missure ; ' in the rest of the class a part called ' corpus callosum' 

 is added, which completes the connecting or ' commissural ' ap- 

 paratus. 



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 being brought forth prematurely, 

 as compared with the rest of the class. 



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

 may be termed, from its cerebral character, Lyencephala, — 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 cere- 

 bellum (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 ' an- 

 terior venae ') terminate in the right auricle. The squamosal in 

 many, retain their primitive separation as distinct bones. The 



