SOME INTERNAL STRUCTURES 73 



In a number of skulls belonging to various genera 

 of Delphinidae with numerous teeth, Professor Kiiken- 

 thal found here and there that the regular arrangement 

 of the relative positions of teeth in the upper and 

 in the lower jaw was lost. The regular arrangement 

 is that the teeth of the two jaws should alternate an 

 obviously convenient arrangement for the due pre- 

 hension of the fish or octopuses upon which they 

 feed ; alternating teeth would be better able to lay 

 hold of this slippery food. When this accurate cor- 

 respondence ceases it is brought about by the inter- 

 calation of teeth - - a proceeding which naturally 

 increases the total number. If this process is going 

 on now, there is nothing unreasonable in thinking 

 that it has been going on in the past in corres- 

 pondence, perhaps, with the increase in length of the 

 jaws themselves. Thus the number of teeth in 

 dolphins is greater now than it has been. They 

 are, therefore, to be derived from creatures with fewer 

 teeth, so far more like the typical mammalia. Another 

 argument pointing in the same direction is afforded 

 by the ancient Zeuglodonts, treated of more fully on 

 another page. (See p. 308.) 



These Cetaceans had a dentition conforming in 



O 



number of teeth to the more typical mammalia. 

 Their teeth were also more conformable to those 

 of the mammalia generally in their heterodonty ; but 

 we shall recur to this after considering the traces 

 of heterodonty still remaining in the group of 

 whales. 



Having dealt generally with the number of teeth 



