DIVISIONS OF MAMMALIA. 55! 



surface of the cerebral hemispheres is generally thrown into 

 numerous convolutions. (Cetacea, Carnivora, Sirenia, Probos- 

 cidea, Ungulata, Quadrumana.} 



d, Archencephala, characterised by the fact that the cerebral 

 hemispheres now completely overlap the cerebellum and olfac- 

 tory lobes ; the number of convolutions attains its maximum ; 

 and there is a corpus callosum. (Man.) 



This is the primary classification of the Mammalia put forth 

 by Owen, and there can be no question but that in many 

 respects it expresses substantial and important differences. It 

 will not be adopted here, partly because it is somewhat difficult 

 to follow or to apply in practice, and partly because some of 

 the characters upon which it is founded are denied by other 

 eminent naturalists. Thus, in the definition of the sub-class 

 Lyeucephala it is stated as one of the essential characters that 

 there is no corpus callosum or commissure between the hemi- 

 spheres of the cerebrum. On the other hand, it is asserted 

 by Flower and Huxley that a corpus callosum does exist in 

 these animals, though it never attains to any high degree of 

 development. 



III. It was proposed by De Blainville, and the arrangement 

 has been accepted by Huxley and Rolleston, to divide the 

 Mammalia into the following three sub-classes, founded upon 

 the nature of the reproductive organs : 



a. Ornithodelphia, characterised by the fact that the uterine 

 enlargements of the oviducts do not coalesce even in their 

 inferior portion to form a common uterine cavity, but open 

 separately as in the Birds and Reptiles. Furthermore, the two 

 uteri open, not into a distinct vagina, but into a cloacal cavity, 

 into which the rectum and ureters also discharge themselves ; 

 so that the condition of parts is very much the same as it is 

 in Birds. 



This division includes only the Duck-mole (Ornithorhynchus) 

 and the Porcupine Ant-eater (Echidna), forming collectively 

 the single order of the Monotremata. 



b. Didelphia, characterised by the fact that the uterine 

 dilatations of the oviducts continue distinct throughout life, 

 opening into two distinct vaginae, which in turn open into a 

 urogenital canal, which is distinct from the rectum, though 

 embraced by the same sphincter muscle. 



This sub-class contains the Marsupialia, such as the Kan- 

 garoos, Opossums, Wombats, &c., most of which are almost 

 entirely confined to Australia. They have many other char- 

 acters in common, which will be spoken of hereafter. 



III. Monodelphia, characterised by the fact that the uterine 



