64 STORY OF THE AMPHIBIANS 



seems liiglilj probable that this is true, and that the 

 reptiles, which we shall discuss next, branched off 

 independently at about the same time or at least 

 very low down on the amphibian stem. Hence both 

 are very ancient. 



Classification 



This class of vertebrate animals is characterized 

 by two states of existence — one aquatic the other ter- 

 restrial — at least by a larval form in the egg or out 

 of it. Eggs are always formed and true limbs are 

 always indicated at some stage of life. The eggs are 

 small, the body (now) uncovered, and the skull joins 

 the back-bone by tivo ball-and-socket joints as in the 

 mammals ; but the lower jaw is hinged to the skull 

 by special bones, which is not the case in mammals. 

 In these last only the lower jaw is hinged directly to 

 the floor of the skull. The following is a key to the 

 orders of the amphibians, both fossil and living ; then 

 follows a key to the tailed forms, and finally there is 

 a key to our common frog-forms in the Eastern United 

 States. 



