c;2 THE COMMON FROG. [chap. 



Lias for fossil allies of our Frog, we shall (judging by 

 what we yet know) fail to find any not at once refer- 

 able to one or other of the three ordinal groups above 

 enumerated. 



Fossil frogs and toads have as yet only been found 

 down to the miocene, the oldest being some found in 

 the so-called " brown coal " which is not a carbon- 

 iferous deposit at all. The remarkable thing, however, 

 is that the difference between these oldest known 

 Frogs and the existing forms is so very trifling. They 

 are as complete and thorough frogs as any that live 

 now. 



Again, the fossil Urodeles similarly resemble their 

 existing representatives, and no one extinct species 

 exhibits characters in any way tending to bridge over 

 the chasm which separates the Urodela from the 

 A noui'a. 



When, however, we descend to the Lias, Trias, and 

 Carboniferous rocks, we come upon a great variety of 

 extinct species of animals evidently allied to those 

 forming the three Batrachian classes already described. 

 They form, however, an order by themselves, to which 

 the term Labyrinthodo7tta has been applied, and thus 

 our search into the past has brought us a rich and 

 important harvest, and has introduced us to the fourth 

 and last order belonging to the Frog class of verte- 

 brate animals. The Labyrinthodonts were creatures 

 with long tails and mostly two pairs of limbs, but 

 these members were always relatively small with 

 slender toes. Some species attained a greater size by 

 far than does any existing Urodela, even the gigantic 

 Salamander. 



