XII PHYLUM CHORDATA 523 



The skull has two condyles in connection with the atlas, 

 instead of the single condyle of the reptiles and birds ; 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 vestiges, which unite with the scapula in the adult. 



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, 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 distinguishable by differences of shape 

 into incisors, canines, and grinding teeth, i.e., are heterodont ; 

 in some instances the teeth are all alike (homodont). A 

 cloaca is absent except in the Prototheria. 



A movable plate of cartilage, the epiglottis, overhangs the 

 glottis or passage 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 alimentary 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. The heart is completely divided into two halves, 

 a right and a left, between which there is no aperture of 



