312 Dwellers of the Sea and Shore 



head. Its complete outer covering Is granular, resem- 

 bling the rough, rasping hide of the shark. It was, 

 in fact, taken for a form of that animal when first 

 found. The deception was made the more easy on 

 account of its sharklike tail. But when this creature 

 was living, the time of the shark was not yet. In 

 tracing this animal through the next succeeding series 

 of rocks, we find it becomes more fishlike and the 

 rough skin has for the most part given way to tough, 

 horny, and in some cases bony, plates; in short, our 

 creature becomes a mailed fish. Then we find that it 

 becomes very numerous, many families can be recog- 

 nized, and in some of them the armor has become quite 

 highly specialized. Large overlapping plates invest 

 the head and trunk, and the flexible hind body and tail 

 are covered with hard scales. One dorsal fin was the 

 rule among these armor-bearing fishes, but appended 

 to the sides in the place of the pectoral fins of present- 

 day forms was a pair of ponderous flippers, not unlike 

 those of a sea turtle. Yet all this accouterment did not 

 prevent their extinction, if indeed they may be said to 

 have been exterminated by the attacks of enemies, for 

 they disappeared entirely when the sharks began to 

 dominate the waters of the sea. 



The coal age unquestionably saw a remarkable in- 

 crease in the numbers and in the variations of the forms 

 of animals. This is as true of those that lived on the 

 land as well as those that inhabited the sea. Many old 

 types were dying out, but they were succeeded by others 

 better fitted to cope with the changing conditions of the 

 globe. In point of numbers of individuals, however, 

 the crinoids, corals, brachiopods, and shell-secreting 



