SOME INTERNAL STRUCTURES 69 



incisors, canines, and molars. Another characteristic 

 feature of mammalian dentition will be postponed 

 until after the actual dentition of adult whales has been 

 described and compared with that of other mammals. 



Broadly speaking, it is correct to define the toothed 

 whales as mammals in which there is no specialisation 

 of the teeth ; but there are some slight exceptions, 

 which will be dealt with presently. The number, size, 

 and position of the teeth of the Odontocetes varies, 

 but the majority have a large number of smallish, 

 conical teeth embedded in both upper and lower jaws. 

 The actual numbers vary much ; the greatest number 

 are seen in the genus Inia, where no less than 221 are 

 reckoned up. As will be seen in the account of the 

 different kinds of whales, the number of the teeth is 

 often made use of as a generic character. Among 

 the Delphinidae there are a gradual series of genera, 

 in which the number of teeth gets reduced. It must 

 not be imagined, however, that we can actually start 

 from some such form as Inia, with abundant teeth, 

 and derive from it the various genera in which the 

 teeth are reduced> and arrange those genera in the 

 order of this reduction. But it will be convenient to 

 take them in such an order. 



Through a gradual reduction in the number we 

 arrive at the genus Delphinapterus (the Beluga), 

 where there are but nine teeth on each side of each 

 jaw. In Grampus this dentition is still further re- 

 duced ; the teeth in the upper jaw have disappeared 

 altogether, and there are only a few, three to seven, 

 on each side of the lower jaw, arranged near to the 



