PLACENTAL AND NON-PLACENTAL MAMMALIA. 129 



may be termed the placental or truly viviparous, and the non- 

 placental or ovo-viviparous Mammalia. The truly viviparous Mam- 

 malia are the most numerous ; and are principally distinguished 

 by the mode of their development. They do not come into the 

 world until they are provided with all their organs ; and, before 

 their birth, they derive their support from a net- work of mater- 

 nal blood-vessels, named the placenta. It is also remarked, that 

 their brain is more perfect than that of the ovo-viviparous 

 Mammals ; its two hemispheres being united together by a large 

 commissure, or band of connecting fibres, termed the corpus cal- 

 losum ( 96) ; lastly, the walls of the abdomen are never sup- 

 ported by bony projections fixed on the edge of the pelvis, such 

 as we shall see in the second grand division of this class. The 

 Mammalia, thus organised, differ very much amongst themselves 

 in the general conformation of their bodies ; some being adapted 

 to inhabit the air, as the Bats ; others resembling Fishes, as thd 

 Whales ; whilst the ordinary Mammalia are formed to live more 

 or less completely on the earth. 



118. In the non-placcntal or ovo-viviparous Mammalia, on 

 the other hand, there are several points of structure which show 

 that they are to be regarded as an inferior group intermediate 

 between the higher group whose characters have just been given, 

 and the Oviparous Yertebrata. The young are born in an ex- 

 tremely imperfect state, closely resembling that of the young of 

 the other Mammalia when they come into the world very pre- 

 maturely ; and as they do not remain long enough within the 

 interior of the parent to give time for the formation of the pla- 

 centa, they are properly distinguished by a term which expresses 

 its absence. The term ovo-viviparous is not so correct a desig- 

 nation, in a scientific point of view ; since it applies more pro- 

 perly to those animals just alluded to ( 116), in which the ani- 

 mals are really oviparous, but in which the young are born alive, 

 by the retention and hatching of the eggs within the body of the 

 parent : but it serves to express, in a popular manner, that this 

 division is intermediate between the truly viviparous Mammals, 

 and the truly oviparous classes of Vertebrata, which is shown to 

 be the case in many points of their structure. Of these, the two 



