FOSSIL FlHHEb. 



V6 



wounds. "With the exception of that portion of the base of the 

 spine inserted in the body, which was smooth, the external surface is 

 enamelled with " gauoine " and is always more or less highly orna- 

 mented or sculptured with longitudinal ridges, striae or tubercles 

 (as in Ctenacauthus, etc.), or transversely and spirally decorated 

 (as in Gyracanthus). The spines of Gyracanthus are unsym- 

 metrical in form and worn down at their extremities. They were 

 no doubt attached to the pectoral fins. 



The spines of PJdestes (Fig. 13) are so strongly serrated as to have 

 led their first discoverer (Prof. Leidy) to conclude that they were 

 part of a fish's jaw with teeth. The series of " Ichthyodorulites" 

 in Wall-case 1 has all been derived from the Carboniferous Lime- 

 stone and from the Coal-measures. 



Fig. 14.— Upper jaw of Port-Jackson Shark, Cestracion Philippi (recent), showing variation 



in forms of teeth in same jaw. 



In the Carboniferous Limestone and Coal-measures we also find 

 an immense variety of forms of palatal teeth with which the mouths 

 of these ancient Selachians were armed, like that of the modern 

 Cestracion Philippi, 



The existence at the present day of the Cestracion Philippi, com- 

 monly known as the ' Port-Jackson Shark,' has been of the greatest 

 assistance to palaeontologists, for it has thrown the most important 

 light upon the detached fossil teeth which are so frequently met 

 with in strata of Cretaceous, Jurassic, and Carboniferous age. 



