364 CLASSIFICATION OF VERTEBRATES. 



Except in the monotremes, which are provided with a horny 

 beak, the mouth of the mammals is bounded by movable lips 

 near the margins of the jaws ; between the lips and the jaws on 

 either side are the cheeks, and in many apes and rodents these 

 are developed into large cheek-pouches, sometimes hairy on the 

 inner surface. A tongue is always present. In the cetacea it 

 is immovable, in all others it is mobile and sensory, and in the 

 giraffe it is even prehensile. At its base it bears the papillae 

 circumvallatas, special aggregations of taste buds. Beneath 

 the tongue is a single or double under-tongue or sublingua, 

 especially developed in the insectivores, which may be the ho- 

 mologue of the tongue of the lower vertebrates, the functional 

 tongue being a new formation. Three pairs of salivary glands 

 are present (parotid, submaxillary, and sublingual) except in the 

 carnivorous cetacea. The secretion is most abundant in the her- 

 bivorous mammals. A soft palate is alway present, behind 

 which are the choanae. 



The teeth show a greater range of variation than is found in 

 any other group of vertebrates. They are lacking in but few 

 forms, as Echidna, Mams, and the baleen whales, while in the 

 adult Ornithorhynchus they are replaced by cornified teeth, al- 

 though true teeth are present in the young. They never have 

 such an extensive distribution as is found in reptilia and ichthy- 

 opsida, but are confined to the margins of maxillary, premaxil- 

 lary, and mandibular (dentary) bones. In all cases they are 

 thecodont, i.e., are situated in sockets or alveoli, although, as in 

 the dolphins, the sockets may run together into a continuous 

 groove, while in some shrews the molars in the adult are firmly 

 anchylosed to the jaws. 



In a few cases, as in the denticete whales, the teeth are all 

 similar in form (homodont), but usually they are differentiated 

 (heterodont) into incisors, canines, and molars. The incisors 

 are placed in the premaxilla, 1 and in a corresponding position in 

 the lower jaw. The canines are placed behind the suture sepa- 

 rating maxillary and premaxillary bones, and never exceed one 

 on a side in either jaw. Behind the canines are the molars. In- 

 cisors and canines have single roots, and the crown is usually a 



1 In some armadillos the premaxillary teeth cannot be incisors. 



