550 MANUAL OF ZOOLOGY. 



CHAPTER LXXIII. 



CLASSIFICATION OF THE MAMMALIA. 



NUMEROUS classifications of the Mammalia have been pro- 

 posed, and it is a matter of regret that no one has been 

 universally accepted by Zoologists. Here, it will be sufficient 

 to describe briefly the three leading systems upon which the 

 Mammalia have been divided into sub-classes ; whilst the first 

 will be adopted as sufficient for all practical purposes. 



I. By many writers the class Mammalia is divided into two 

 great primary divisions, the Placentalia or Placental Mammals, 

 and the Implacentalia or Non-placental Mammals, according 

 as the structure known as the " placenta " is present or absent. 

 The placenta, as before said, is a vascular organ developed in 

 the greater number of Mammals, by means of which the blood 

 of the foetus is brought into relation with the blood of the 

 mother.* The sub-class Placentalia, in which such a vascular 

 connection between the mother and foetus exists, comprises by 

 far the largest number of the Mammals. The sub-class Im- 

 placentalia, in which no such vascular connection exists, com- 

 prises only the two orders of the Monotremata and the Mar- 

 supialia. 



II. By Professor Owen the Mammalia are divided into four 

 sub-classes, characterised by the structure of the brain, as 

 follows : 



a. Lyencephala, characterised by the fact that the cerebral 

 hemispheres are usually without folds, and leave the cerebel- 

 lum, the olfactory lobes, and part of the optic lobes uncovered. 

 The hemispheres are not connected together by a corpus cal- 

 losum. (Monotremata and Marsupialia). 



b. Lissencephala, characterised by the fact that the cerebral 

 hemispheres are smooth or are provided with few folds, and 

 leave the cerebellum and part of the olfactory lobes exposed. 

 A corpus callosum is present. ( Chewoptera, Insectivora, Rodentia^ 

 Edentata.} 



c. Gyrencephala, characterised by the fact that the hemispheres 

 of the cerebrum cover the greater part of the cerebellum and 

 the olfactory lobes. A corpus callosum is present, and the 



* No traces of vascular prominences comparable to the Mammalian pla- 

 centa occur in any animals below the rank of Mammals, except in some Sharks 

 and some Ascidians. In the Sharks, however, the vascular eminences are 

 developed from the umbilical vesicle, and not, as in Mammals, from the 

 allantois. 



