INTRODLXTION. 57 



generally, and to propose an arrangement en- 

 tirely different from those of his predecessors, 

 the characters of -which are principally taken 

 from the form and structure of the scales. His 

 orders are as follows : — 



I. Placoidians — So named from the irregu- 

 larity presented by the solid parts of their cover- 

 ing, composed of masses of enamel. It includes 

 many fossil forms, the Sharks, and Rays. 



II. Ganoidiatis — Containing varied forms, the 

 Sauroid fishes, Siluri, Sturgeons, &c., charac- 

 terized by scales of an angular form, composed 

 of two substances, plates of horn or bone, placed 

 one upon another, and covered by a thick layer 

 of enamel. 



III. Ctenoidians — The common character is in 

 the thin plates forming the scales, being pecti 

 nated or toothed on their posterior edge, which 

 makes them feel rough to the touch. It con- 

 tains the Chetodons, Pleuronectes, the Percoid 

 fishes, &c. 



VI. Cycloidians — They have the scales formed 

 of simple plates, those of the lateral line with 

 a tube for the transmission of the lubricating 

 mucus. It includes the Mullets, Salmon, Cy- 

 prini, &c.* 



* If we estimate the number of fish now known, to 

 amount to about 8000, we may state that more than three- 



