652 GEOLOGY 



leaves, give a distinctly xerophytic aspect to the overgrowth made 

 up of lepidodendrons, sigillarias, calamites, and cordaites. This 

 is not the case with the undergrowth, but this would not be ex- 

 pected of shaded plants. The force of the inference from the xero- 

 phytic aspect of the overgrowth is, however, much weakened by 

 the fact that the vegetation of undrained swamps and bogs assumes 

 many of these xerophytic features. It is clear that a more critical 

 study of the problem is necessary before a final conclusion concern- 

 ing the climate of the period is reached. 



II. THE LAND ANIMALS 



Amphibians, insects, spiders, scorpions, and myriapods, lived 

 on the land at this time. The amphibians are perhaps of chief 

 interest, for they were the first of the -great line of land vertebrates, 

 the ruling dynasty from that day to this. So far as the evolution 

 of air-breathing vertebrates is concerned, this is one of the most 

 important periods in geological history. 1 



The rise of the amphibians. Tracks attributed to amphibians 

 are found in the Devonian and the Mississippian, but in neither of 

 these systems have any bones of these . animals been found in 

 America, and only imperfect ones in Europe. When the fossils of 

 amphibians first appear in abundance, in the later Coal Measures, 

 they were already so differentiated as to imply a long antecedent 

 existence. Most of them were rather primitive in structure, but 

 they were genuine amphibians, not transition forms. In the < 

 of many of them, fossils representing all stages of growth have 

 been found, showing that the young had gills, and that the gills 

 were lost later in life. All of them seem to have had elongate 

 forms of salamandrine aspect, and their heads were well roofed 

 over by the bony plates of the skull. On account of tins last 

 feature they are called stegocephalians (roof-headed). Some of 

 them have also been named labyrinthodonts, from the ini rieate in- 

 folding of the dentine of their teeth. The group varied in length 

 and strength of limb, in agility, ability to climb, etc. The elon- 



1 Williston. The Faunal Relations of the Early Vertebrates, Jour. <;<.!. 

 1909. 



