68 A BOOK OF WHALES 



TEETH 



Whales are, as is well known, divisible into two 

 groups those with and those without teeth, the 

 Odontoceti and the Mystacoceti of various authors. 

 The Mystacoceti, however, the " whalebone whales," 

 possess teeth in the young condition, while there are 

 plenty of instances of the commencing disappearance 

 of teeth among the Odontoceti. Thus the line which 

 separates the two divisions of existing whales is not 

 so hard and fast as was stated before recent dis- 

 coveries in the growth of the teeth of these animals. 



Before considering the growth of the teeth, how- 



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ever, it will be well to lay briefly before the reader the 

 principal facts in the structure of the teeth of existing 

 toothed whales. 



A very marked feature of their teeth is the charac- 

 teristic "homodonty." This term, it should be ex- 

 plained, is applied to teeth when the whole series is 

 composed of teeth which are alike. In most mammals 

 there is what is known as Heterodonty, i.e., the teeth 

 are specialised in different directions. Thus in man 

 there are the anterior incisors, cutting teeth, which are 

 different in form and in function from the posterior 

 cheek teeth, molars or crushing teeth. The differ- 

 entiation is more emphasised still in some other 

 animals, less so again in others. But on the whole 

 the mammals stand apart from all other vertebrate 

 animals by the fact of their Heterodonty. The teeth 

 of a frog, of a snake, or of a lizard, are all more 

 or less alike ; there is no possibility of speaking of 



