MOUTH OF MAMMALS. 



383 



CHAPTER XXX. 



ALIMENTARY CANAL AND APPENDAGES OF MAMMALS. 



§ 223. Mouth. — Fleshy lips form the main characteristic of the 

 mammalian mouth. But they are wanting in the Monotremes, 

 with other significant shortcomings of mammalian excellence. 

 Lips are, here, transitorily manifested, it is true, at the suckling 

 period; but soon degenerate into the pergameneous border of 

 the beak in Platypus, and are reduced, in Echidna, to the scarcely 

 movable margin of the small terminal oral orifice of the adult. 

 The Cetacea show the greatest extremes within the limits of a 

 natural group in the development of the lips. They are barely 

 represented in the Porpoise, fig. 297, and other Delphinida by 



297 



Section of mouth and nose, Porpoise. 



the low, firm, ridge of integument, supported by adipo-fibrous 

 tissue with scarce a trace of ' orbicularis oris ' : while in the 

 Whale (Balama) the upper lip falls down like a thick curtain 

 some feet in depth concealing the baleen^ and overlapping the 



