58 MICROSCOPIC OBJECTS. 



scale are a constant indication of the nature and cha- 

 racters of the fish to which it belongs. The apphcation 

 of this knowledge to the fossil remains of these animals 

 has already produced abundant fruit, while the more 

 recent discovery of fish -scales in flint, bids fair to extend 

 our acquaintance with the finny tribes which were coeval 

 with the diiferent strata of the earth's crust. 



Professor Agassiz has formed the class of fishes into 

 four orders, a,ccording to the structure of their covering, 

 as follows: Enamelled Scales. — 1. Placo'idians, Fish 

 characterized by having their skin irregularly covered 

 with plates of enamel; in some genera these are large, in 

 others they are small, almost points : — Examples, shagreen 

 on the skin of some sharks, and the prickly toothlike 

 tubercles on the skin of rays. This order includes all 

 the cartilaginous fishes of Cuvier, except the sturgeon. 

 — 2. Gano'idians. The fishes of this order have angular 

 scales composed of horny or bony plates covered with 

 enamel ; fifty out of sixty genera are extinct : — Examples, 

 sturgeons and bony pike. — Scales not enamelled. — 

 3. Ctendidians. This order of fishes have the scales ser- 

 rated, or notched, on their posterior fi^ee edges : — Ex- 

 ample, the perch. — 4. Cycloid fishes have smooth scales, 

 simple at their margin, composed of layers, which give 

 the outer surface an ornamental appearance : — Example, 

 the herring, salmon, &c. 



The scales of fish are formed fi'om a secretion similar 

 to that from which the epidermis or scarf-skin of animals 

 is secreted. This secretion takes place periodically in the 



