72 A BOOK OF' WHALES 



most teeth the most primitive, and have they given 

 rise to those with a reduced dentition ? Or is the 

 converse true ? Or finally, is it safest to take the 

 middle path and make two series, one ascending and 

 one descending? Are, for instance, dolphins with a 

 moderate number of teeth nearest to the ancestral 

 form from which have arisen by multiplication on the 

 one hand the Inia, and by reduction the Narwhal ? 

 This supposition agrees in some ways more nearly 

 with what we know of mammalian dentition in general. 

 It has been pointed out that the typical mammalian 

 dentition is heterodont. It is also limited in numbers, 

 and those numbers are definite. Apart from the 

 Marsupials (in which, moreover, fifty-six is the 

 greatest number of teeth) and a very few other 

 instances, no mammal has or had more than forty- 

 four teeth. Even here there is nothing like the 

 abundance of teeth of Inia or Platanista. Further- 

 more, the numbers of teeth of the many-toothed 

 dolphins appear to be not exactly fixed to a tooth 

 or two ; whereas in the mammalia, as a rule, with 

 but few exceptions (such as Priodon, an Armadillo, 

 and the Manatee), the number does not vary, except, 

 of course, in occasional abnormalities. 



On a priori grounds, therefore, (dangerous grounds 

 sometimes on which to build an aroaiment intended to 



o 



last !), we should be rather disposed to regard the 

 excessive dentition of the typical dolphins as not a 

 primitive state of affairs, but one derived from some- 

 thing more nearly approaching to what is characteristic 

 of mammals in general. 



