570 ZOOLOGY SECT. 



pair of incisors of the upper jaw is present only in the Hares and 

 Rabbits ; the number of pre-molars and molars varies from — 



2 3 3 



and they may develop roots. 



In the Insectivora the dentition is heterodont, complete, and 

 diphyodont. AH the teeth are' rooted. There are never fewer than 

 two incisors on either side of the lower jaw. The canines are not 

 of large size. The crowns of the molars are beset with pointed 

 tubercles. 



In the Chiroptera the dentition is complete, and the teeth are 

 all rooted. There is a milk-series which differs entirely from the 

 permanent teeth. In the insectivorous Chiroptera (Bats) the 

 molars are provided with pointed cusps, while in the frugivorous 

 forms (" Flying Foxes ") they are longitudinally grooved or 

 excavated. 



In the Primates the teeth are heterodont and diphyodont, aud 

 always form roots. There are almost invariably two incisors on 

 each side in each jaw, and, in all but the Hapalidse, three molars. 

 The dental formulae of the various families have been given in 

 the synopsis of the classification. The dentition of Man differs 

 from that of the rest of the order in the teeth forming a continu- 

 ous series not interrupted by a diastema, and in the comparatively 

 small size of the canines. 



The mouth in Mammals is bounded by fleshy lips. On the floor 

 of the mouth is situated the tongue, which is usually well 

 developed, but varies in size and shape in different orders. Its 

 surface is covered with papillae of different forms, in association 

 with certain of which are the special end-organs of the nerves of 

 taste — the taste-huds. The roof of the mouth is formed in front 

 by the hard palate, consisting of the horizontal palatine plates of 

 the maxillary and palatine bones covered with mucous membrane. 

 Behind the hard palate projects backwards the soft muscular fold 

 of the soft palate, also with taste-buds, which divides the cavity of 

 the pharynx into two chambers, an upper and a lower. In front 

 of the opening leading from the lower division of the pharynx 

 into the larynx, is a cartilaginous plate — the epiglottis— of which 

 rudiments only are found in certain lower Vertebrates. 



The cesophagus is always a simple straight tube. The stomach 

 varies greatly in different orders, being sometimes simple, as in 

 the majority of Mammals, sometimes divided into chambers, as in 

 the Cetacea and the Ruminants. 



In the majority of Mammals the stomach is a simple sac, as in 

 the Rabbit (p. 460). But in certain groups it is complicated by 

 the development of internal folds, and may be divided by con- 

 strictions into a number of different chambers. The complication 



