7 6 THE LESSON OF EVOLUTION 



our seas by the sharks and rays. The primitive 

 Elasmobranchs, called Ichthyotomi, are limited to the 

 Deutozoic era. In them the notochord was formed 

 by a cylindrical rod of equal thickness, sheathed with 

 cartilage, which was very slightly calcined and was 

 not constricted into vertebrse. Also the pectoral fins 

 had a segmented axis, like those of the Dipnoi. The 

 early Selachii, from which all our sharks and rays are 

 descended, have the sheath of the notochord more 

 calcified and constricted in the centre of each ver- 

 tebra. The pectoral fins have no segmented axis, 

 and the skin is covered with thorny scales. Of 

 these, the sharks and dogfish, in which ossification 

 of the vertebral column takes place chiefly in radiat- 

 ing plates, appeared in the Carboniferous period, and 

 at once became common ; but they had no sharp teeth 

 in the jaws, only blunt crushing teeth on the palate. 

 The Rays, in which ossification of the vertebrse takes 

 place chiefly in concentric plates, although first known 

 in the Carboniferous, remained rare for a long time, 

 and only became abundant in the Cainozoic era. But 

 the most interesting group of Elasmobranchs is the 

 Acanthodii, in which the basal cartilages of the 

 pectoral fins are much shortened ; the rays are 

 arranged like those of ordinary fishes, and the scales 

 are enamelled. These became extinct at the end of 

 the Deutozoic era. 



The sub-class Teleostomi, which contains all the bony 

 fishes, appears first in the Devonian as Crossoptery- 

 gians, with their paired fins formed by a long seg- 

 mented axis, fringed on both sides with rays, like the 

 vane of a feather. They attained their maximum in 

 the Devonian, and then slowly declined, being at the 



