AMPHIBIANS OF THE FAR PAST 59 



far back as frogs are found in the rocks, tliej are all 

 frogs and their tadpoles can be recognized even as 

 differing from others. The fossils of all living am- 

 phibians are in rocks that are modern compared with 

 those in which rest the monsters of the class. Hence 

 there has been plenty of time for the fossil forms to 

 degenerate into the kinds now living. 



Fig. 25.— Slab of sandstone with amphibian footprints, from coal- 

 measures of Pennsylvania, x Vs. 



But no fossil among those giants approaclied the 

 form of a frog. No reptile or amphibian which can be 

 recognized as such is found fossil away down where 

 fishes are so abundant. Just under the coal period 

 some amphibians show, and just above it some reptiles. 



