96 



KINSHIPS OF SHARKS 



Affinities 



In concluding the present chapter, the probable affini- 

 ties and interrelationships of the Elasmobranchs may be 

 summarized as follows (v. Fig. 103) : — 



1. Of all known stems that of the shark is most nearly 

 ancestral in the line of jaw-bearing vertebrates. 



2. A generalized form not unlike Cladoselache might 

 well represent the ancestor of Pleuracanthid, as well as 

 of the primitive Cestraciont, of Acanthodes, and of the 

 modern sharks and rays. 



Fig. 102 A. — The mantis, or devil ray, Ccpkaloptera {Dicerobatis) draco. X ^. 

 (After Gf^NTHER.) Tropical seas. 



3. On the evidence of the Permian Pleuracanthids, 

 lung-fishes (Dipnoans) and the earliest bony fishes 

 (Crossopterygians) are to be derived from an advancing 

 shark type. 



4. From the ancestral stem of the recent sharks 

 Cestracionts were the most early differentiated : it is one 

 of their more generalized forms, Cestracion, that has alone 



