4 86 



HISTORICAL GEOLOGY 



It is hard to point out briefly all the differences between amphibians 

 and reptiles, and it is indeed sometimes extremely difficult, if not im- 

 possible, to tell from the skeleton alone of extinct forms to which 

 class a specimen belongs. One important difference, however, is 

 to be found in the articulation of the skull to the backbone, which in 

 amphibians is by means of two knuckle-like projections of bone 

 (condyles) and in reptiles and birds by one. In fishes there is no 

 movable articulation. 



All Carboniferous amphibians belonged to the Stegocephali (Fig. 

 461) (Greek, stege, roof, and cephale, head), in which the skull was 





g*£m 





,^W5 



A/V 



Fig. 461. — A Permo-Carboniferous landscape. The characteristic vegetation ; two 

 figures of the amphibian Eryops upon the land, each about seven feet long, are shown ; 

 a reptile {Limnoscelis) in the water; and a gigantic dragon fly in the air. (After 

 Prof. S. W. Williston.) 



covered with bony plates, and the teeth were conical with walls that 

 were sometimes highly infolded (Fig. 462). The limbs of most were 

 weak and adapted more for crawling than for carrying the body well 

 above the ground. 



Carboniferous amphibians varied greatly in size and shape, some 

 attaining a length of almost eight feet. One characteristic genus, 

 Eryops (Figs. 463 and 461), had a rather large, broad, flat head, and 



