THE HISTORY OF ANIMAL LIFE 403 



The trilobites are the most common of fossil Arthropoda. None 

 of this group exists today. There were numerous sorts of trilobites. 

 One must have been concerned in the origin of the king crab, for 

 the immature modern king crab resembles a trilobite in many re- 

 spects (Fig. 222). 



Fossil fishes are not rare. Their study has thrown a great deal of 

 light upon the origin, distribution, and relationship of modern 

 fishes. An ancient order of Amphibia, the Stegocephala, were the 

 first of the air-breathing animals. Unlike modern Amphibia, the 

 bodies of some of these forms were scaly. But of all fossil forms the 

 most bizarre, the most gigantic, and the most diverse in form and 

 habitat were the reptiles. Today their remains make the most im- 

 posing of museum exhibits. Some were adapted to a marine habitat, 

 some were waders and swamp forms, and some had the power of 

 flight. They composed three orders: The Ichthyosauria, aquatic 

 forms; the Dinosauria, enormous animals that lived in swamps or 

 near water; and the Pterosauria, whose fore limbs were modified 

 for flight; one type had a wing spread of twenty feet. 



Ancestry of Birds. The developmental processes and many of 

 the adult characters of birds and reptiles show distinct resem- 

 blances. On these grounds alone the conclusion is justified that 

 modern birds represent a modification of a reptilian-like form. What 

 amounts to confirmation of this conclusion is found in the fossil 

 records. The latter portion of the Mesozoic Era was the period of 

 dominance of the reptilian group. It was during this period that 

 the Pterosauria, or Pterodactylia, flying reptiles, flourished. The 

 wings of these forms, modified from the vertebrate fore limb, con- 

 tained a long fourth digit, the first three digits remaining distinct 

 (Fig. 223). In some modern birds the first three digits develop 

 during embryonic life, but subsequently are obscured. Furthermore, 

 in this era are found the remains of the first bird, Archaeopteryx 

 (Fig. 224). While distinctly a bird, this form also had many rep- 



