126 HIGH STANDING OF THE PLACOIDS. 



such, ranked by Cuvier among the CJiondropterygi% whereas 

 Agassiz places them in his ganoid order. Many of the ex- 

 tinct fishes, too, such as the Acanthodeij Dipteridce, CephalaS' 

 pidce, were, as we have seen, cartilaginous in their internal 

 framework, and yet true ganoids notwithstanding. The prin- 

 ciple of Agassiz's classification wholly dijSTers from that of Cu- 

 vier and the older ichthyologists ; for it is a classification 

 founded, not on the character of the internal, but on that of 

 the cuticular or dermal skeleton. And while to the geologist 

 it possesses great and obvious advantages over every other, — 

 for of the earlier fishes very little more than the cuticular 

 skeleton survives, — it has this further recommendation to the 

 naturalist, that (in so far at least as its author has been true 

 to his own principles), instead of anomalously uniting the 

 highest and lowest specimens of their class, — the fishes that 

 most nearly approximate to the reptiles on the one hand, and 

 the fishes that sink furthest towards the worms on the other, 

 — it gathers into one consistent order all the individuals of 

 the higher type, distinguished above their fellows by their 

 development of brain, the extensive range of their instincts, 

 and the perfection of their generative systems. Farther, the 

 history of animal existences, as recorded in the sedimentary 

 rocks of our planet, reads a recommendation of this scheme 

 of classification which it extends to no other. We find that 

 in the progress of creation the fishes began to be by groupes 

 and septs, arranged according to the principle on which it 

 erects its orders.* The placoids came first, the ganoids suc- 

 ceeded them, and the ctenoids and cycloids brought up the 

 rear. The march has been marshalled according to an ap- 

 pointed programme, the order of which it is peculiarly the 

 merit of Agassiz to have ascertained. 



* We now need to reverse this order, in so far as the placoids and 

 ganoids are concerned, although they may be almost considered — as we 

 think they will be ultimately found to be — contemporaries. — L. M. 



