SYSTEM OF AGASSIZ. 206 



relations of Fishes ;* the scales forming a kind of external 

 skeleton, analogous to the crustaceous or horny coverings 

 of Insects, to the feathers of Birds, and the fur of Quadru- 

 peds, which shows more directly than the internal bones, 

 their adaptation to the medium in which they lived. 



A farther advantage arises from the fact that the ena- 

 melled condition of the scales of most Fishes, w^hich existed 

 during the earlier geological epochs, rendered them much 

 less destructible than their internal skeleton ; and cases fre- 

 quently occur where the entire scales and figures of the 

 Fish are perfectly preserved, whilst the bones within these 

 scales have altogether disappeared; the enamel of the scales 

 being less soluble than the more calcareous material of the 

 bone.f 



* The foundation of this character is laid upon the dermal covering, the 

 skin being that organ wliich, more than any other part of the body, ehowe 

 the relation of every animal to the element in which it moves. 



The form and conditions of the feathers and down show the relation of 

 Birds to the air in which they fly, or the water in which they swim or dive. 

 The varied forms of fur and hair and bristles on the skins of Beasts are 

 adapted to their respective place, and climate, and occupations upon the 

 land. The scales of Fishes show a similar adaptation to their varied place 

 and occupations beneath the waters. 



Mr. Burehell informs me that he has observed, both in Africa and South 

 America, that in the order of Serpents a peculiar character of the scales ap- 

 pears to indicate a natural subdivision ; and that in that tribe, to which the 

 Viper and nearly all the venomous Snakes belong, an acute ridge, or carina, 

 along each dorsal scale may be considered as a distinctive mark. 



t The following are the new Orders, in which M. Agassiz divides the Class 

 of Fishes. 



First Order, PLACOIDIANS. (PI. 27, Figs. 1, 2, Etym. ^A*f, a broad 

 plate.) Fishes of this Order are characterized by having their akin 

 covered irregularly with plates of enamel, often of considerable dimen- 

 sions, and sometimes reduced to small points, like the shagreen on the 

 skins of many Sharks, and the prickly, tooth-like tubercles on the skin of 

 Rays. It compreliends all the cartilaginous fishes of Cuvier, excepting the 

 Sturgeon. 



The enamelled prickly tubercles on the skin of Sharks and Dog-Fisheg 

 VOL. I. — 18 



