relations of Fishes;[1] 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 Quadrupeds, which shows more directly than the internal bones, their adaptation to the medium in which they lived.
A further advantage arises from the fact that the enamelled condition of the scales of most Fishes, which existed during the earlier geological epochs, rendered them much less destructible than their internal skeleton; and cases frequently 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.[2]
- ↑ The foundation of this character is laid upon the dermal covering, the skin being that organ which, more than any other part of the body, shows the relations 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, climate and occupation upon the land. The scales of Fishes show a similar adaptation to their varied place and occupation beneath the waters.
Mr. Burchell informs me that he has observed, both in Africa and South America, that in the order of Serpents a peculiar character of the scales appears 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.
- ↑ The following are the new Orders into which M. Agassiz divides the Class of Fishes.
First Order, PLACOIDIANS (Pl. 27, Figs. 1, 2, Etym. πλαξ, a broad plate.) Fishes of this Order are characterized by having their skin covered irregularly with plates of enamel, often of considerable dimensions, 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 comprehends all the cartilaginous fishes of Cuvier, excepting the Sturgeon.
The enamelled prickly tubercles on the skin of Sharks and Dog-Fishes
