154 HISTORICAL PALAEONTOLOGY. 
Rays, and Chimere (the Elasmobranchii). The majority of 
the forms here alluded to are allied not to the true Sharks and 
=: 
——— 
b 
Fig. 105.—A, Polypterus, a recent Ganoid fish; B, Osteolefzs, a Devonian Ganoid ; 
aa, 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 applied to 
these ; and we have evidence of their past existence in the 
Fig. 106.—//oloptychius nobilissimus, 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. 
