TEETH OF CETACEA. 143 



The sharp-edged teeth perform the office of 

 cuttmg and dividing the yielding textures pre- 

 sented to them ; they act individually as wedges 

 or chisels ; but when co-operating with similar 

 teeth in the opposite jaw, they have the power 

 of cutting like shears or scissors. The flat teeth, 

 of which the surfaces are generally rough, are 

 used in conjunction with those meeting them in 

 the opjjosite jaw for grinding down the food by 

 a lateral motion, in a manner analogous to the 

 operation of mill-stones in a mill. The tuber- 

 culated teeth, of which the surfaces present a 

 number of rounded eminences, corresponding to 

 depressions in the teeth opposed to them in the 

 other jaw, act more by their direct pressure in 

 breaking down hard substances, and pounding 

 them, as they would be in a mortar. 



The position of the teeth in the jaws has been 

 another ground of distinction. In those Mam- 

 malia which exhibit the most complete set of 

 teeth, the foremost in the row have the sharp- 

 edged or chisel shape, constituting the blades of 

 a cutting instrument; and they are accordingly 

 denominated incisors. The incisors of the u})per 

 jaw are always implanted in a bone, intermediate 

 between the two upper jaw bones, and called 

 the intermaxillary bones.* The conical teeth, 



* Those teeth of the lower jaw which correspond with the 

 incisors of the upper jaw, are also considered as incisors. In 

 Man, and in the species of quadrumana tliat most nearly re- 



