CHAPTER XXIX 



THE CLASS ELASMOBRANCHII OR SHARK-LIKE 



FISHES 



HE Sharks. The gap between the lancelets and the 

 lampreys is a very wide one. Assuming the primi- 

 tive nature of both groups, this gap must represent the 

 period necessary for the evolution of brain, skull, and elaborate 

 sense organs. The interspace between the lampreys and the 

 nearest fish-like forms which follow them in an ascending scale 

 is not less remarkable. Between the lamprey and the shark 

 we have the development of paired fins with their basal attach- 

 ments of shoulder-girdle and pelvis, the formation of a lower 

 jaw, the relegation of the teeth to the borders of the mouth, 

 the development of separate vertebrae along the line of the 

 notochord, the development of the gill-arches, and of an ex- 

 ternal covering of enameled points or placoid scales. 



These traits of progress separate the Elasmobranchs from 

 all lower vertebrates. For those animals which possess them, 

 the class name of Pisces or fishes has been adopted by numerous 

 authors. If this term is to be retained for technical purposes, 

 it should be applied to the aquatic vertebrates above the lam- 

 preys and lancelets. We may, however, regard fish as a popular 

 term only, rather than to restrict the name to members of a class 

 called Pisces. From the bony fishes, on the other hand, the 

 sharks are distinguished by the much less specialization of the 

 skeleton, both as regards form and substance, by the lack of 

 membrane bones, of air-bladder, and of true scales, and by 

 various peculiarities of the skeleton itself. The upper jaw, for 

 example, is formed not of maxillary and premaxillary, but of 

 elements which in the lower fishes would be regarded as belonging 



to the palatine and pterygoid series. The lower jaw is formed 



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