154 



HISTORICAL PAL.EONTOLOGY. 



Rays, and C/iimcercB (the Elasmohranchii). The majority of 

 the forms here alluded to are allied not to the true Sharks and 



Fig. 105. — A, Polypterus, a recent Ganoid fish; B, Osteolepis, a Devonian Ganoid; 



a a, Pectoral fins, showing the fin-rays arranged round a central lobe. 



Dog-fishes, but to the more peaceable " Port Jackson Sharks," 

 with their blunt teeth, adapted for crushing the shells of Mol- 

 luscs. The collective name of " Cestracionts " is apphed to 

 these ; and we have evidence of their past existence in the 



Fig. 106. — Holoptychius fiobilisshmts, restored. Old Red Sandstone, Scotland. 

 A, Scale of the same. 



Devonian seas both by their teeth, and by the defensive spines 

 which were implanted in front of a greater or less number of 

 the fins. These are bony spines, often variously grooved, 

 serrated, or ornamented, with hollow bases, implanted in the 

 integument, and capable of being erected or depressed at will. 



