HISTORY. 21 



Impressed with the fact that the peculiar scales of Poly- 

 pterus and Lcpidosteus are common to all fossil osseous fishes 

 down to the chalk, he takes the structure of the scales 

 generally as the base for an ichthyological system, and distin- 

 guishes four orders : — 



1. Placoids. — Without scales proper, but with scales of 

 enamel, sometimes large, sometimes small and reduced to 

 mere points (Eays, Sharks, and Cyclostomi, with the fossil 

 Hybodontes). 



2. Ganoids. — With angular bony scales, covered with a 

 thick stratum of enamel : to this order belong the fossil 

 Lepidoides, Sauroides, Pycnodontes, and Coelacanthi ; the 

 recent Polypterus, Lepidosteus, Sclerodermi, Gymnodontes, 

 Lophobranches, and Siluroides ; also the Sturgeons. 



3. Ctenoids. — With rough scales, which have their free 

 margins denticulated : Chsetodontidse, Pleuronectidse, Percidas, 

 Polyacanthi, Sciaenidse, Sparidse, Scorpsenidse, Aulostomi. 



4. Cycloids. — With smooth scales, the hind margin of 

 which lacks denticulation : Labridae, Mugilidse, Scombridae, 

 Gadoidei, Gobiid8e,Mur8enid8e,Lucioidei, Salmonidse, Clupeidae, 

 Cyprinidee. 



We have no hesitation in affirming that if Agassiz had 

 had an opportunity of acquiring a more extensive and inti- 

 mate knowledge of existing fishes before his energies were 

 absorbed in the study of ^ their fossil remains, he himself would 

 have recognised the artificial character of his classification. 

 The distinctions between cycloid and ctenoid scales, between 

 placoid and ganoid fishes are vague, and can hardly be main- 

 tained. As far as the living and post-cretacean forms are 

 concerned, the vantage-ground gained by Cuvier was aban- 

 doned by him ; and therefore his system could never supersede 

 that of his predecessors, and finally shared the fate of every 

 classification based on the modifications of one organ only. 

 But Agassiz has the merit of having opened an immense new 



