508 



MAMMALIA. 



permanent, and post-permanent, respectively, the present state 

 of our knowledge with regard to their occurrence may be ex- 

 pressed by the subjoined table, in which v indicates vestigial 

 and / functional dentitions. 



The vestiges on the labial side of the permanent molars clearly belong 

 to the milk series (which in Diphyodonts erupt in the anterior part of the 

 mouth). The presence of these vestiges raises the question as to whether 

 there is any morphological distinction between the premolar and molar 

 series. For the homology of the replaced tooth of marsupials the reader 

 is referred to the systematic account. 



In most lower Vertebrata the succession of teeth appears to be indefi- 

 nite, and except in Teleostei all the teeth are formed, as in mammals, 

 from a tooth-band, the inner end of which continues to grow and produce 

 new buds on the lingual side of those last formed. In the Teleostei the 

 successive teeth appear to be in many cases formed independently from 

 the buccal epithelivun and not from a tooth-band. 



Alimentary Canal. In addition to the hard structures at 

 the entrance to the digestive cavity, soft moveable lips which 

 bound the mouth opening, and a fleshy tongue which is of very 

 various form and lies on the floor of the buccal cavity, are of 

 special importance for the prehension and preparation of the 

 food (Fig. 267). In the Monotremata the lips are replaced by 

 the edges of the beak. The tongue, however, is never absent, 

 but it may be immoveable, and completely fused with the floor 

 of the mouth, as in the whales. Its front part is mainly tactile 

 in function, but in some cases it is used to seize (giraffe) or cap- 

 ture food (ant-eaters). Variously shaped papfllae, which are 

 often cornified and bear recurved hooks, project from its upper 



