346 



GENERAL ZOOLOGY 



Some species grew to be as large as alligators. Their skulls 

 (Fig. 172) were solidly roofed over with bone. Their teeth, 

 in many cases, showed complicated foldings of the enamel ; 

 hence they have been called labyrinthodonts, i.e. labyrinth- 

 toothed (Fig. 173). There is little doubt that the labyrinth- 

 odonts descended from the fishes. Anurans and urodeles did 

 not appear till later. Both are probably descended from the 

 labyrinthodonts, each by its own line of ancestry. 



The close of the 

 Age of Amphibi- 

 ans is marked by a 

 great change in the 

 topography of North 

 America, for it was 



Appalachian system 

 of mountain ranges 

 was uplifted. This 

 brought a great area 

 of land in eastern 

 North America above 

 FIG. 173. Section of Tooth of Labyrinthodont ^e l eve l o f the sea, 



(From Baskett's Story of the Amphibians and as jg shown on the 

 Reptiles) . _ . x ^ T 



map (Fig. 174). No 



species of animal of earlier time is known to have existed 

 after this upheaval. The causes underlying the culmination 

 and decline of so many species at this time are not thor- 

 oughly understood. They are probably connected with the 

 changes already spoken of, with the decline in the temper- 

 ature, and with the slow removal of carbon dioxide from 

 the atmosphere. 



Paleozoic Time : Era of the Ancient Forms of Life. The 

 geological periods so far considered since the close of Archrean 

 Time (see pp. 295, 324, and 345) are usually grouped under 



