THE VERTEBRATA 147 



Let us return now to the Vertebrate. A char- 

 acter common to all the groups of the Verte- 

 brata is the possession of teeth. Readers of the 

 previous volumes of this series will recollect that, 

 even among birds, instances of the possession of 

 teeth may be found among fossil forms, although 

 they are absent in the birds of the present day. 

 In all the other divisions of the Vertebrata, the 

 presence of teeth is the rule, their absence an ex- 

 ception so rare that we may easily note the chief 

 instances of it. Among Amphibia, there are 

 Toads that have no teeth ; among Reptiles, the 

 Tortoises and Turtles have none ; among Mam- 

 mals, teeth are wanting in Echidna, the Spiny 

 Ant-eater; and in the Ant-eaters and the Whale- 

 bone Whales they are absent in the adult, al- 

 though present in early embryonic life. 



The majority of people, if asked to give a 

 definition of the meaning of teeth, would reply 

 that they are hard structures that grow in the 

 jaw. But this is an idea that requires very con- 

 siderable modification from a scientific point of 

 view. In the first place, they are found in other 

 places besides the jaws ; and in the second place, 

 they are by rights structures originally belonging 

 to the skin. Both these important facts must be 

 illustrated by reference to the Fishes, which ex- 

 hibit the primitive types of teeth. 



In fishes, not only are teeth found on the jaw- 

 bone, but sometimes also on other bones which 

 border upon the cavity of the mouth ; they are 

 found on the palatine bone, or roof-plate of the 

 mouth, and, still more strange, upon bones which 

 belong to the " hyoid apparatus," or skeleton of 

 the gills (see above). The latter may form a set 

 of throat-teeth, which are used for grinders, while 



