THE ALLIES OF THE FROG: AMPHIBIA 345 



those who do not understand it, the animal is a most inter- 

 esting object for study. There is no truth in the oft-repeated 

 statements of its poisonous qualities, except in so far as the 

 acrid secretion of the skin, which is a protection to many 

 amphibians, might be injurious if it gets on sensitive sur- 

 faces, such as the lining of the eyelids. The dark-colored 

 eggs are laid in long strings, like a string of beads, in shal- 

 low water, usually in April, in the latitude of New York. 

 They hatch in two or three weeks into small black tadpoles, 

 which pass through their transformation within about two 

 months. 



Geological Development of Amphibians. As time passed on 

 from the Age of Fishes a large portion of the great interior 

 sea of North America, shown on the map, page 295, became 

 a region of swamps, though many times depressed and exposed 

 to the inroads of the sea. These conditions brought about the 

 formation of the great coal-beds of the continent, alternating 

 with beds of shale, sandstone, and clay. When the land was 

 above the surface of the 

 water, vegetation flour- 

 ished in more than tropi- 

 cal luxuriance ; when the 

 land was depressed, the 



ocean-waters rolled in 



FIG. 172. Skull of Labyrmthodont 



and destroyed the life of 



the land. The swamps of that time are the coal-beds of 

 to-day, and the period is known as the Carboniferous Age 

 or Age of Coal Plants. Though fishes and many invertebrate 

 forms were abundant, progress had been made over pre- 

 ceding periods in the development of back-boned creatures 

 adapted to breathing air. These were amphibians, and from 

 the prevalence of species of this order the period is often called 

 the Age of Amphibians. There were snake-like forms without 

 limbs, and forms with every degree of limb-development. 



