INTRODUCTION. 57 



generally, and to propose an arrangement en- 

 tirely different from those of his predecessors, 

 the characters of which are principally taken 

 i^om 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. Ganoidians 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. 



YI. Cydoidians 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- 



