CHAPTER XIV 
THE CLASS ELASMOBRANCHII OR SHARK-LIKE 
FISHES.—TRUE SHARKS 
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 vertebre 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 lim- 
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 pterygo'd series. The lower jaw is formed 
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