386 HISTORICAL GEOLOGY 



of them had more substantial bony frames and were of larger 

 size (Fig. 409). Certain degenerate types had lost the use of 

 the limbs and doubtless adopted the habits of snakes. 



FIG. 409. A large amphibian of crocodile-like form and habits, as it prob- 

 ably appeared in life. 



First appearance of the reptiles. Recently the bones of 

 true reptiles have been discovered in the Coal Measures of 

 Illinois and Pennsylvania. Not until the next period, how- 

 ever, does the class come into prominence, and so the discus- 

 sion of them is deferred until that Chapter is reached. 



Climate of the Pennsylvania!!. The abundance of vege- 

 tation in the coal swamps has been thought to indicate that 

 North America and Europe were covered with tropical jungles 

 in which the growth of plants was luxuriantly rapid. This 

 would imply a climate warmer than that of the present, and 

 perhaps moister. 



By others, however, it is pointed out that the largest accu- 

 mulations of peat are now being formed in cool regions, such 

 as Canada and northern Europe. Singularly enough, the 

 microscopic structure of the leaves of the coal trees is much 

 more like that of our northern conifers and other hardy plants 

 than like the delicate and thin-skinned leaves prevalent in 



