TEETH OF KEPTILES AND FISHES. 



175 



like the duck, upon the water-insects, shell-fish, and aquatic 

 plants, that it obtains from the mud, into which it is continu- 

 ally plunging its singular bill; and its jaws, entirely destitute 

 of teeth, are famished with horny ridges, by which it can in 

 some degree masticate its food. 



187. Among Birds, there is an entire absence of teeth; 

 and the mechanical division and the reduction of food is per- 

 formed in the stomach, in the manner hereafter to be men- 

 tioned ( 200). The mouths of almost all Reptiles, excepting 

 the Turtle tribe, are furnished with numerous teeth (fig. 

 104) ; but these are not 

 adapted for much variety of 

 purposes, being principally 

 destined to prevent the 

 escape of the prey which 

 the animals have secured ; 

 and their shape is conse- 

 quently nearly uniform, being for the most part simply 

 conical. There are some Lizards, however, which are herbivo- 

 rous ; and these have large rough teeth, somewhat resembling 

 the molars of Mammalia. The Iguanodon, an animal of this 

 tribe, attained a gigantic size in past ages of the world. 



188. In Fishes, the teeth are commonly very numerous (fig. 

 105), but they have for their object only to separate and retain 



Fig. 104. HEAD OF GAVIAL. (Crocodile 

 of the Ganges.) 



Fig. 105. HEAD OF SHARK. 



their food ; and there is little variety in their form. Fre- 

 quently they have no bony attachment, being only held by 

 the gum, as in the Shark ; and they are consequently often 

 torn away, but they are as readily replaced. Sometimes, bow- 



