536 



GEOLOGY. 



The number of spines preserved is exceptionally great when com- 

 pared with the teeth and dermal ossicles, which are about equally 

 well adapted to preservation. The proportion is very much greater 

 than that of later periods, including the present, and implies that the 

 sharks bristled with spines to a degree not retained in later times. 

 The loss of spines in later ages, like the loss of the armor-plates and 

 other clumsy defensive devices, may be assigned to replacement by 

 agility, intelligence, and more effective weapons of attack, as New- 

 berry has suggested; but their great development at this time doubt- 

 less had its special reason in the fact that most of the sharks were 

 only provided with pavement teeth, which were ineffectual weapons 

 against those sharks that were armed with piercing and cutting teeth, 

 and hence the former had especial need for a bristling array of spines 

 as a defense against their aggressive kin. 



Sharks with piercing teeth were represented at the very opening 

 of the period by Cladoselache, Fig. 239, and later by Cladodus and allied 



Fig. 239. — Cladoselache fyleri Newb. Restoration by Dean about one-fifth natural 

 size. From Cleveland Shales, Ohio. Often referred to the Devonian. (After 

 Dean.) 



genera which were numerous and wide-spread. Some of these were 

 large and formidable fishes, " armed with teeth in many rows, several 

 hundred in each set." (Fig. 235, t and u). The petalodonts were a 

 conspicuous group of sharks with peculiar cycloid cutting teeth. The 

 arthrodirans, which were abundant and gigantic during the transition 

 period from the Devonian, lingered in reduced importance through 



