178 



HISTORICAL GEOLOGY 



the standpoint of the evolution of the air-breathing Vertebrates 

 the Pennsylvanian is regarded as the most important period in geo- 

 logical history. It is to be remembered that the earliest Amphib- 

 ians almost certainly evolved from certain types of Fishes. All 

 Mississippian and Pennsylvanian Amphibians are often classed 

 together as Stegocephalians, so-called because of the relatively 

 large, bony, roof-like plates of the skulls. 



As regards the principal forms of Amphibians of Pennsylvanian 

 time, the writer can do no better than to quote an excellent sum- 



Fig. 108 



A Pennsylvanian Amphibian (Labyrinthodont), Eryops. This creature at- 

 tained a length of 6 or 8 feet. (Courtesy of the American Museum of 

 Natural History.) 



mary by S. W. Williston: 1 "The predominating types of the 

 Pennsylvanian were what we usually call the Branchiosaurs and 

 the Microsaurs, for the most part small or very small creatures, at 

 least as small as their nearest living relatives of the present time, 

 the Salamanders. We are quite justified in the belief that their 

 habits in general were not greatly unlike these descendants, rather 

 sluggish creatures living about or in the water, for the Branchio- 

 saurs at least passed through larval stages. They were more or 

 less protected by an external bodily armor against their enemies, 

 whether of their own or other kinds, in all probability terminat- 

 ing their existence as distinctive types long before the close of 

 the Paleozoic. But among them there were some classed with the 



1 Outlines of Geologic History, 1910, p. 164. 



