THE DEVONIAN PERIOD 



461 



Insects. — No undoubted re- 

 mains of insects have been found 

 in strata of this or earlier periods, 

 although their discovery may be 

 expected at any time. 



Fishes 



The appropriateness 

 term Age of Fishes as 



Fig. 437. — Stylonurus, a gigantic De- 

 vonian eurypterid, some of which were eight 

 feet long. (After Clarke and Ruedemann.) 



of the 

 applied 

 to the Devonian Period is evi- 

 dent when the importance of 

 this great class, not only in the 

 life of that time but as the prob- 

 able progenitors of all subse- 

 quent vertebrate life, is con- 

 sidered. Fish were the rulers of 

 the Devonian seas and rivers, 

 perhaps even to a greater degree 

 than were the trilobites in the 

 Cambrian and the cephalopods 

 in theOrdovician. The fact that 

 fish were abundant during the period does not imply that other forms 

 of life were less abundant than in previous periods. For example, 

 brachiopods are exceedingly common fossils in almost all Devonian 

 strata, while in many of the rocks of this age fish fossils are extremely 

 rare and a search of many days may not be rewarded by even a 

 fragment. 



Ostracoderms. — One of the strangest classes of Devonian animals 

 was the ostracoderm (Greek, ostrakon, shell, and derma, skin). These 

 were fishlike in shape but were probably not even closely related 

 to fishes. The description of one well-known member of the class 

 (Cephalaspis, Greek, cephale, head, and as-pis, shield) gives a general 

 notion of this group (Fig. 438 A). The most striking feature was the 

 crescent-shaped plate which covered the head and fore part of the body. 

 Besides the protection afforded by this head shield, the tail was covered 

 with rhomboidal scales. The eyes were situated close together on the 

 top of the head. The lower jaw, if it ever existed, has not been found. 

 Although many hundreds of the bony parts have been found, no in- 

 ternal skeleton has been discovered, and it is therefore probable either 

 that none existed or that it was cartilagenous and was consequently 



