120 Biology in America 



appearance very early upon the stage of life, for the more 

 primitive the creature the earlier its appearance in the strata 

 of the earth. Thus we find ancient lungfishes in the Silurian, 

 and in the Devonian, the period succeeding, they attained 

 their greatest prominence; while the sharks, which in many 

 respects are most primitive of the true fishes of the present, 

 appeared at about the same time as the lungfishes, but did 

 not attain dominance in the seas until much later. 



The ancient lungfishes, like the Otetracoderms, were heavily 

 armored; some possessed powerful crushing or tearing jaws, 

 and some may have attained a size of twenty-five feet and a 

 gape of four feet. 1 



The fossil sharks include many forms of peculiar interest. 

 One of these from the Devonian period Cladoselache gives 

 strong evidence in its paired fins for the lateral fin fold theory, 

 as noted in the preceding chapter. The vertebra lacked 

 centra, resembling in this respect the cyclostomes and dip- 

 noans. Perhaps nowhere else have fossil forms reproduced 

 so faithfully their structure as in this shark, for Dean has 

 shown that sections of its muscles, magnified one thousand 

 times, showed very clearly the finer structure of the muscle 

 substance (cross striations and sarcolemma). The modern 

 shark is covered with teeth-like scales known as shagreen 

 denticles, from the shagreen leather made from the skin of 

 the shark. From these scales have arisen on the one hand 

 teeth which are covered with enamel, and on the other scales 

 and plates in which this is lacking. Large scales or bony 

 plates are common in the skin of many fishes. We have 

 already seen them in the ostracoderms and the armored fishes, 

 but they are absent in modern sharks. In fossil forms how- 

 ever they were frequent and closely resembled those of the 

 fossil lungfishes, which were probably cousin to the sharks. 



The evolutionist is accustomed to thinking of life as ever 

 changing. Yet there are some forms which have come down 

 to us unchanged through untold ages. The shark Cestracion 

 is such an one. It roamed the seas in early Mesozoic days, 

 the dawn of the "age of reptiles," possibly 100,000,000 or 

 more years ago. 



The Australian lungfish Neoceratodus has persisted with 

 but small change for an even greater length of time, while the 

 Foraminif era, Orbulina and Globigerina, whose shells compose 

 much of the ooze on the sea bottom, are found as far back as 

 the Ordovician, perhaps 200,000,000 to 300,000,000 years ago, 

 and possibly existed long before. Living on some small 

 islands off the coast of New Zealand is the last representative 

 'I am assuming here the somewhat problematical relationship of 

 the Arthrodira to the Dipnoi. 



