SOME INTERNAL STRUCTURES 71 



case it is the lower jaw which has become edentulous. 

 A second series of modifications is seen amono- the 



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Physeteridae, the Cachalot, and the Ziphioid whales. 



The Cachalot has functional teeth only in the 

 mandible, where they form a row of strong conical 

 teeth ; but in addition to these are a series of smaller 

 teeth in the upper jaw, which are not to be seen 

 in the dried skull, as they are not embedded in the 

 bone, but only in the gum, which naturally is stripped 

 off or decays away in the course of preparation of the 

 skull for museum purposes. This kind of reduction 

 is still further exaggerated in the Ziphioid whales. 

 In all of these the number of teeth actually used is 

 very limited, not more than two pairs usually one 

 pair, and those are limited to the lower jaw. But 

 in addition to these there are in most, if not in all, 

 Ziphioid whales a set of smaller teeth only in the 

 upper jaw or in both jaws, which are like the cor- 

 responding teeth of the Cachalot embedded only in 

 the gum, and so are, as a rule, lost in skulls acquired 

 by museums. These teeth are clearly on the wane ; 

 and as even the teeth of the lower jaw are sometimes 

 not extruded, and in other cases lost before the animal 

 dies, it is evident that these whales are not so very 

 far removed from the whalebone whales ; but it should 

 be observed that they exhibit no trace of the com- 

 pensating whalebone. 



So much then for the General modifications of the 



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teeth, as regards numbers, which are exhibited in the 

 series of toothed whales. 



The question arises, Are those whales with the 



