462 



HISTORICAL GEOLOGY 



incapable of fossilization. In another genus (Bothriolepis) a pair of 

 appendages encased in bony plates, somewhat as are the appendages 

 of a lobster, extended from the sides of the head. Ostracoderms 

 seldom attained a size greater than 6 or 7 inches. 



Certain inferences as to the habits of ostracoderms can be drawn 

 from their structure. They probably lived on the sea bottom as did 

 the trilobites, either burrowing in the mud above which only their 

 eyes and their dorsal shield showed, or because of their dull coloring 

 crawling over it inconspicuously. The fact that they were protected 

 implies that they had to contend with enemies more powerful than 

 themselves. They lived in large numbers in certain localities, as is 



Fig. 438. — Devonian ostracoderms: A, Cephalaspis, about six inches long; 

 B, Bothriolepis, about seven inches long. 



shown by the great abundance of their shields which form thin beds 

 in some places and are said to be hardened by the oil from their re- 

 mains. 



Ostracoderms began in the Ordovician, reached their climax in the 

 Devonian, and became extinct at its close. 



Sharks. — Sharks lived in the Devonian in considerable numbers, 

 but since their skeletons were cartilagenous the fossil evidence of 

 their existence consists largely of teeth, spines (which probably stood 

 in front of the dorsal fin), and small bony denticles which were doubt- 

 less embedded in the skin. The best known and most simple in struc- 

 ture of these ancient sharks (Cladoselache, Fig. 439 A) varied from 

 two to six feet in length. It had a short, blunt snout with the mouth 

 situated on the lower side but farther front than in the modern shark. 



