262 



PALEOZOIC TIME. 



Fig. 499 represents a spine from the fin of a Shark (from Ontario 

 County, N. Y.), which was originally at least ten inches long. Figures 

 502, 504, illustrate the positions of such spines. 1 



Fig. 499. 



Fin-spine of a Shark, Machseracanthus sulcatus ( X % )• 



Other remains of sharks are the teeth or bones of the mouth. 

 The masticating apparatus in some of the ancient sharks, as in the 



Figs. 502-512. 



Selachians. — Fig. 502, Spinax Blainvillii (x%); 503, Spine of anterior dorsal fin, natural size; 

 504, Cestracion Philippi (X%); 505, Tooth of Lamna elegans ; 506, id. Carcharodon angusti-- 

 dens; 507, id. Notidanus primigenius ; 508, id. Hybodus minor: 509, id. Hyb. plicatilis ; 510 

 Mouth of Cestracion, showing pavement-teeth of lower jaw ; 511, Tooth of Acrodus minimus ; 

 512, id. Acrodus nobilis. 



modern Cestracion of Australia (Fig. 504, reduced), was a pavement 

 of bony pieces; Fig. 510 shows the pavement of the lower jaw of 



1 In several genera of Selachians, the dorsal fin is armed at its anterior margin with 

 a large spine. In the genus Spinax (Fig. 502, reduced), there are such spines, one to 

 each dorsal fin; Fig. 503 represents one of natural size for a fish (Spinax) about 2} feet 

 long. — Such spines exist also in the Cestracionts (Fig. 504), the Hybodonts, and the 



