THE PENNSYLVANIAN PERIOD 



179 



heterogeneous group which we call Microsaurs, which had made 

 a very distinct advance, both toward a higher existence and away 

 from the water. . . . Some lost the dermal armor completely 

 and became fleet of movement, as is evidenced by the structure 

 of the limbs, limbs mimicking in form and structure so closely 

 those of modern quick-run- 

 ning Lizards as to be prac- 

 tically indistinguishable." 



The Labyrinthodonts , 

 and certain other closely 

 related forms, comprised 

 another important group 

 of Pennsylvanian Amphib- 

 ians. They are so named 

 because of the peculiar, 

 labyrinthine, internal 

 tooth-structure (Fig. 109). 

 They were the gigantic 

 land Vertebrates of the 

 period, some having 

 reached a length of 7 or 8 

 feet (Fig. 108). 



Reptiles. — Whether or not true Reptiles existed in this period 

 depends largely upon the classification of the primitive land Ver- 

 tebrates. The abundance of true Reptiles in the succeeding (Per- 

 mian) period strongly suggests their earlier differentiation from 

 the Amphibians. According to Williston: "We may be assured 

 that some of them (Amphibians) , before the close of the Pennsyl- 

 vanian, were inhabitants of high-and-dry land regions where fleet- 

 ness of movement, rather than obscurity, preserved them from their 

 enemies, crawling Reptiles in everything save some insignificant 

 technical details of their plates." 



Fig. 109 

 Transverse section of a Labyrinthodont 

 tooth. (After Owen from Norton's 

 "Elements of Geology," by permission 

 of Ginn and Company, Publishers.) 



