536 



The Earth and Its hihabitaiits Chajige unit x 



Fig. 486 (right) Trilobite fossils in liviestone 

 formed early in the fourth era. This gives you 

 some idea of how crowded with trilobites the 

 seas ?nust have been. (u. s. national museum) 



that the Cambrian period is sometimes 

 called the Age of Trilobites. Perhaps by 

 this time you have forgotten just where 

 the Crustacea fit in the classification of 

 animals. In the next few problems vari- 

 ous phyla and classes \\\\\ be mentioned 

 so this would be a good time to review- 

 parts of Unit I. You will find summaries 

 of the classification of animals on pages 

 6$-66 and of plants on pages 90-9 1 . 



Imagine that another 150 million years 

 have passed; we are still looking at the 

 earth in the fourth era (the Paleozoic), 

 but it is toward the close of the fourth 

 era. The period is the Carboniferous, the 

 period in which the coal beds were 

 formed. In this period there must have 

 been enormous swamps in many parts 

 of the globe with a thick growth of 

 ferns and fernlike plants. Although tree 



ferns are still to be found in South 

 America and on southern Pacific islands, 

 most ferns of the present time are small 

 annual plants. Most of the Carboniferous 

 ferns were trees, forming dense forests. 

 Fish abounded in the ocean, and amphib- 

 ians, like our newts and salamanders. 



l'"Ki. 487 This is a photogrciph of a reconstruction of a forest of the Carboniferous 

 period. Ferns varied from small plants to huge trees. The animal on the tree at the 

 left is a cockroach, (field museum of natural history) 



