176 Professor Owen, on the [Jan. 27, 



system. These, indeed, were early suggested by the results of the 

 application of the characters employed by Cuvier in the formation of 

 the primary and secondary groups of the class ; the sloth, for example, 

 was placed above the horse, the mole above the lynx, and the bat above 

 the dog : even the Ornithoi'hynchus paradoxus — shown by accurate 

 anatomical scrutiny to be the most reptilian of the mammalian class — 

 takes precedence of the colossal and sagacious elephant in the Cuvierian 

 scheme. 



The question of the truly natural and equivalent primary groups 

 of the class had been kept in view by the speaker whenever oppor- 

 tunities occurred of adding to the knowledge of tlie anatomy of the 

 mammalian class, and especially when engaged in dissecting any of 

 the rarer forms which died at the gardens of the Zoological Society of 

 London, and in which the structure of the brain could be unravelled. 



The facts so acquired gradually impressed a conviction that the 

 modifications of the cerebral organ were those which most truly indicated 

 the primary groups of the mammalia. 



Prior to the year 1 836, the brain in Mammalia was supposed to 

 differ from that in all other vertebrate animals by the presence of the 

 large mass of transverse white fibres, called ' corpus callosum,' by the 

 anthropotomist ; which fibres, overarching the ventricles, and diverging 

 as they penetrate the substance of either hemisphere of the cerebrum, 

 bring every convolution of the one into communication with those of 

 the other hemisphere, whence the other name of this part — the * great 

 commissure.* In that year Prof. Owen discovered that the brain of 

 the kangaroo, the wombat, and some other marsupial quadrupeds, 

 wanted the ' great commissure ; ' and that the cerebral hemispheres 

 were connected together, as in birds, only by the ' fornix ' and ' anterior 

 commissure.* Soon afterward, he had the opportunity of determining 

 that the same deficiency of structure prevailed in the Ornithorhynchus 

 and Echidna. Since many other modifications of structure, more or 

 less akin to those characterizing birds and reptiles, were found to 

 be associated with the above oviparous type of brain, together with 

 some remarkable peculiarities in the economy of reproduction, he 

 then had suggested that the Mammalia might be divided into ' placental* 

 and ' implacental.' 



Impressed, however, with the fact that such binary division, like 

 that which might be based upon the leading differences of dentition, 

 was too unequal to be natural, and other modifications of the mam- 

 malian brain being found to be associated with concurrent conditions 

 of other organs, the speaker had subsequently proposed a four-fold 

 primary division of the Mammalia, based upon four leading types or 

 conditions of the cerebral structure in that class. 



The first or lowest modification is that in which the cerebral 

 hemispheres of small relative size and simple exterior, are connected 

 together only by the anterior commissure and fornix. 



The second is that in which the great commissure or * corpus 

 callosum ' is superadded, without any other advance in the develop- 



