The Story of the Rocks 



123 



tion was forming the future beds of coal, conditions were 

 ripe for the advent of new types of life upon the earth, and 

 here occurred the birth of a new creation. What were the 

 creatures that linked the fishes to the amphibians Ave do not 

 know, but we have already noted indications of a change in 

 * the ancient lung fish and crossopterygian. 



The earliest traces of amphibians known are some "foot- 

 prints on the sands of time" left for our information in the 

 sandstone of the late Devonian period by Thinopus, and pre- 

 served for our further instruction in the museum of Yale 

 University, to which reference has already been made in the 

 preceding chapter. 



The creatures which marked the coming of the new race 

 were many in number and varied in kind. Some were great 

 in length if not in stature, measuring up to seven or eight 

 feet, while others were minute, not exceeding a few inches 

 in size. Some were limbed and others limbless, some had 



A Stegocephalan 



After Williston, "Water Keptiles of the Past and Present' 

 (adapted from Neumayr). 



By permission oj the University 0} Chicago Press. 



simple teeth, while in others the tooth was infolded from the 

 sides producing so complex a pattern as to have given to one 

 group the name of labyrinthodont. This character is also 

 found in one of the crossopteiygian fishes, the Devonian 

 Holoptychius, a further indication of a possible relationship 

 between them and the early amphibians. They all however 

 possessed in common the heavy complicated coats of mail upon 

 the head which has given the group its name of Stegocephala. 

 In these early armored forms as well as in many recent ones 

 (i. e., sturgeon and other ganoid fishes) the animal carried 

 much of his skull, like a helmet, on the surface of his head. 

 This was developed in the skin, while underneath was a casing 

 for the brain developed in cartilage. In higher forms the 

 dermal bones have sunk below the surface becoming intimately 

 united in development with the cartilaginous brain case, the 

 whole forming a compact structure in which in the adult it 



