PALEOZOIC ROCKS AND ERA. 319 



Amphibians. The introduction of amphibians must 

 be regarded as a great step in the progress of life ; for 

 they are the first true land vertebrates and air-breathing 

 vertebrates. Yet we must remember, on the one hand, 

 that amphibians, as their name implies, all of them at 

 some period of their life, some of them permanently, 

 breathe both air and water both by gills and by lungs ; 

 and on the other hand we must remember that Ganoid 

 fishes also supplement their gill-breathing by lung- 

 breathing. The amphibians are intermediate between 

 fishes and true reptiles. They are represented now by 

 frogs, toads, newts, etc,, 



Now, in the Carboniferous, and long afterward, am- 

 phibians were very diiferent from any of those mentioned 

 as still living. They belonged to a peculiar order now 

 long extinct, called Labyrinthodonts, from the labyrin- 

 thine structure of their teeth (Fig. 252). All the living 

 amphibians are small creatures ; these were often of huge 

 size. All the living kinds have soft, moist skin ; these 

 were partly covered with 

 large, ganoidal plates. 

 The early Ganoids, too, 

 had the same labyrin- 

 thine structure of the 

 teeth, though less 



FIG. 251. Structure of a ganoid tooth. (After 



marked (Fig. 251). In Agaesiz.) 



fact, the transition from 



the reptilian Ganoids to the ganoidlike amphibians of 



the coal-measures is so gradual that it is difficult in some 



cases to say whether some of these are amphibian reptile 



or ganoid fish (Fig. 253). 



Amphibians seem to have been very abundant in the 

 coal-measures some snakelike forms, with very small 

 or no feet, some lizardlike forms, some fishlike forms, 

 and some huge crocodilian forms, but not with croco- 

 dilian affinities. These huge forms were, however, more 



