CHARACTERS DERIVED FROM SCALES. 269 
horny coverings of Insects, to the feathers of 
Birds, and the fur of Quadrupeds, which shows 
more directly than the internal bones, their adap- 
tation 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 geolo- 
gical epochs, rendered them much less destructi- 
ble than their internal skeleton; and cases fre- 
quently occur where the entire scales and figure 
of the Fish are perfectly preserved, whilst the 
bones within these scales have altogether disap- 
peared; the enamel of the scales being less 
soluble than the more calcareous material of the 
bone.* 
* The following are the new Orders into which M. Agassiz 
divides the Class of Fishes. 
First Order, PLACOIDIANS. (PI. 27, Figs. 1, 2, Etym. 
7r\a^, 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 are well known, from the use made of them in rasp- 
ing and polishing wood, and for shagreen. 
Second Order, GANOIDIANS. (PL 27, 3, 4, Etym. 
yavoQ, splendour, from the bright surface of their enamel.) The 
families of this Order are characterized by angular scales, com- 
posed of horny or bony plates, covered with a thick plate of 
enamel. The bony Pike (Lepidosteus Osseus, PI. 27*, Fig. 1); 
