476 ZOOLOGY SECT. 



in the Rabbit, owing to the modifications which they undergo, in 

 order to take part in the formation of the placenta the structure 

 by whose instrumentality the foetus receives its nourishment from 

 the walls of the uterus. The placenta is formed from the serous 

 membrane or chorion the outer layer of the amniotic folds, in a 

 limited disc-shaped area, in which the distal portion of the allantois 

 coalesces with it. The membrane thus formed develops vascular 

 processes the chorionic villi which are received into depressions 

 (the uterine crypts) in the mucous membrane of the uterus. The 

 completed placenta with its villi is supplied with blood by the 

 allantoic vessels. The placenta of the Rabbit is of the type 

 termed deciduate, the villi of the placenta being intimately united 

 with the uterine mucous membrane, and a part of the latter 

 coming away with it at birth in form of a decidua, or after-birth. 



2. DISTINCTIVE CHARACTERS AND CLASSIFICATION OF RECENT 



MAMMALIA. 1 



The Mammalia are air-breathing Vertebrates, with warm blood, 

 and with an epidermal covering in the form of hairs. The bodies 

 of the vertebrae are in nearly all Mammals ossified each from three 

 independent centres, one of which develops into the centrum 

 proper, while the others give rise to thin discs of bone the epi- 

 physes. Also characteristic of the spinal column of Mammals are 

 the discs of fibro-cartilage termed intervertebral discs, which 

 intervene between successive centra. 



The skull has two condyles for connection with the atlas, instead 

 of the single condyle of the Sauropsida; and the lower jaw, which 

 consists of only a single bone on each side, articulates with the 

 skull in the squamosal region without the intermediation of the 

 separate quadrate element always present in that position in Birds 

 and Reptiles. 



Each of the long bones of the limbs is composed in the young 

 condition of a central part or shaft and terminal epipbyses, the 

 latter only becoming completely united with the shaft at an 

 advanced stage. 



In the pectoral arch, the coracoid of the Birds and Reptiles is 

 usually represented only by a vestige or vestiges, which unite with 

 the scapula in the adult. The ankle-joint is always situated 

 between the tibia and the tarsus. 



Mammals are typically diphyodont, i.e., have two sets of teeth 

 a milk or deciduous set, and a permanent set : some are 

 monophyodont, i.e., have only one set. The teeth are thecodont, 

 ie., the base of each tooth is embedded in a distinct socket or 

 alveolus in the substance of the bone of the jaw ; and nearly 



1 Extinct groups are referred to in dealing with the distribution. 



