156 HISTORICAL PALEONTOLOGY. 



rays on both sides, the scales being sometimes rounded and 

 overlapping (fig. 106), or more commonly rhomboidal and 

 placed edge to edge (fig. 105, A). Numerous forms of these 



Fig. Wt.Ptericht/tys cornutus. Old Red Sandstone, Scotland. 

 (After Agassiz.) 



" Fringe-finned " Ganoids occur in the Devonian strata, such 

 as Holoptychius, Glyptolcemus, Osteolepis, Phaneropleuron, &c. 

 To this group is also to be ascribed the huge Onychodus (fig. 

 102, d and e}, with its large, rounded, overlapping scales, an 

 inch in diameter, and its powerful pointed teeth. It is to be 

 remembered, however, that some of these " Fringe-finned '' 

 Ganoids are probably referable to the small but singular group 

 of the "Mud-fishes" (Dipnoi}, represented at the present day 

 by the singular Lepidosiren of South America and Africa, and 

 the Ceratodus of the rivers of Queensland. 



Leaving the Ganoid fishes, it still remains to be noticed that 

 the Devonian deposits have yielded the remains of a number 

 of fishes more or less closely allied to the existing Sharks, 

 Rays, and Chimccrce (the Elasmobranchii*). The majority of 

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

 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 

 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. 

 Many of these " fin-spines " have been preserved to us in the 

 fossil condition, and the Devonian rocks have yielded examples 

 belonging to many genera. As some of the true Sharks and 



