VERTEBRATA: MAMMALIA. 655 



tremata and Marsupialia. The sub- class Placentalia, in which a 

 placenta is present, comprises all the other orders of Mammalia. 



II. The Mammalia may be divided as follows into three 

 primary divisions according to the structure of the reproduc- 

 tive organs. This arrangement was proposed by De Blainville, 

 and is accepted by Huxley, Rolleston, Flower, and other dis- 

 tinguished authorities : 



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-eaters (Echidna}, forming collectively 

 the single order of the Monotremata. 



b. Didelphia, characterised by the fact that the uterine dilata- 

 tions 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 charac- 

 ters in common which will be spoken of hereafter. 



III. Monodelphia, characterised by the fact that the uterine 

 enlargements of the oviducts coalesce to a greater or less 

 extent to form a single uterine cavity, which, however, generally 

 shows its true composition by being divided superiorly into 

 two cornua. The uterus opens again into a single vagina, 

 which is always distinct from the rectum. This sub -class 

 corresponds with the division of the " Placental " Mammals, 

 and includes all the Mammalia except the Monotremes and 

 Marsupials. 



