xlviii Introduction. 



Sub-class, Didelphia. 



The Didelphia are represented by the single order Marsu- 

 ^ialia, which resembles the Monotremata in the possession of 

 ' marsupial ' bones, though in few other points besides those 

 common to all Mammalia. The urogenital canal is much more 

 distinct from the rectum than in the Monotremata, but, as is 

 the case also in certain Monodelpfiia, the external orifices of both 

 canals are embraced by a common sphincter muscle. The testes 

 are never retained in the abdomen, as in the Ornithodeljihia and 

 in some Monodelphia, but are suspended in a scrotum placed an- 

 teriorly to the penis. The young are extruded from the uteri in 

 an imperfect condition of development, and whilst going through 

 the further stages necessary for enabling them to provide for 

 themselves, they are attached to a long mammary nipple, which is 

 ordinarily contained within a marsupial pouch. The coracoid never 

 reaches the sternum ; true teeth are never absent ; the angle of the 

 lower jaw is almost always inflected. 



Sub-class, Monod.elph.ia. 



The Monodelphia, which are also known as ' Placentalia/ differ 

 from the two other sub-classes in the following points : with a few 

 exceptions, such as the Hare amongst the Rodents, and Ori/cteropus, 

 amongst the Bruta, their female generative canals form an azygos 

 corpus uteri of greater or less length, which opens into an azygos 

 vagina ; which, again, with the exception of Bradypus, opens 

 always by a single orifice into a urogenital canal. In all Mono- 

 delphia the vessels of the allantois come into relation with the 

 vessels of the uterus, and the two sets of vessels form a placenta, 

 which has not been observed in the other two sub-classes. Accord- 

 ingly as portions of the maternal structures come away with the 

 foetal elements of the placenta at birth or not, the Monodelpliia 

 are divided into Deciduata and Non-deciduata, the former of 

 these groups corresponding to the Unguiculata , with the exclusion 

 of Manis and probably also of Rhinoceros ; and the latter to the 

 TJngulata and Mutica of Linnaeus. The scrotum is never prepenial 

 as in Marsupials ; the testes are, however, sometimes retained 

 within the abdomen as in Monotremata : and in Centetes, in which 

 this is the case, a tendency to develope an inflection at the angle 



