xm PHYLUM CHORDATA 463 



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

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

 phyodont, 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 helerodont ; in some instances the teeth are all alike (homo- 

 dont). A cloaca is absent as a rule, 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 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 communication. Each 

 half consists of an auricle and a ventricle, opening into one another 

 by a wide aperture, guarded by a valve composed of three mem- 

 branous cusps on the right side, two on the left. The right ventricle 

 gives ofi the pulmonary artery ; from the left arises the single 

 aortic arch, which passes over to the left side, turning round the 

 left bronchus in order to run backwards as the dorsal aorta : it 

 therefore represents the left aortic arch of Reptiles. The blood 

 is warm. The red blood-corpuscles are non-nucleated and usually 

 circular. 



The two cerebral hemispheres, in all but the Monotremes and 

 Marsupials, are connected together by a band of transverse fibres 

 the corpus callosum not represented in the lower Vertebrates. 

 The dorsal part of the mid-brain is divided into four optic 

 lobes the corpora quadrigemina. On the ventral side of the 

 hind-brain is a transverse band of fibres the pons Varolii 

 by which the lateral portions of the cerebellum are connected. 

 An external ear, and a malleus and incus are characteristic. 



The ureters, except in the Prototheria, open into the bladder. 



With the exception of the Monotremes, Mammals are all vivi- 

 parous. The foetus is nourished before birth from the blood- 

 system of the parent through a special development of the foetal 

 membranes and the lining membrane of the uterus, termed the 

 placenta. After birth the young Mammal is nourished for a longer 

 or shorter time by the milk or secretion of the mammary glands of 

 the mother. 



