xin PHYLUM CHORDATA 447 



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 the form of a decidua, or after-birth, 



2. DISTINCTIVE CHARACTERS AND CLASSIFICATION 



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 inter-vertebral 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 

 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 epiphyses, 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. 



Mammals are typically dipliyodont, 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, 

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

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

 always the teeth in different parts of the jaw are clearly dis- 

 tinguishable by differences of shape into incisors, canines, and 

 grinding teeth, i.e., are heterodont; in some instances the teeth are 

 all alike (homodonf). A cloaca is absent except in the Prototheria. 



A movable plate of cartilage the epiglottis represented only 

 by a rudiment in some Amphibia and Sauropsida overhangs the 

 slit commonly termed glottis leading from the pharynx into the 

 cavity of the larynx. 



A partition of muscular fibres usually with a tendinous centre 

 the diaphragm divides the cavity of the body into two parts, an 

 anterior the thorax containing the heart and lungs, and a 

 posterior the abdomen containing the greater part of the ali- 

 mentary canal with its associated glands the liver and pancreas 

 and the renal and reproductive organs. 



The lungs are freely suspended within the cavity of the thorax. 



