SWIMMING nLADDER OF FISHES. 385 



ther, like a tesselated pavement, and reminding us 

 of the arrangements we have seen adopted in the 

 calcareous coverings of the Echinida. 



Some of the cartilaginous fishes are, in like 

 manner, protected by calcareous plates, appended 

 to the integuments. There is a row of plates of 

 this kind, of a quadrangular shape, which pass along 

 the middle of the back in the Sturgeon : and the 

 whole body of the Ostraciou is covered with osseous 

 scales. All these have no immediate relation to the 

 skeleton, but are apparently remnants of inferior 

 types, of which one of the prevailing characters is 

 the external situation of the protecting organs.* 



* The different forms of the scales alford valuable indications of 

 the natural families of fishes ; and their importance in this respect 

 has been ably pointed out by M. Agassiz, who, assuming them as 

 the basis of his system of classification, divides fishes into the four 

 following orders. 



1. Placoides; so named on account of the irregularity of the 

 solid parts of their integuments, which consist of masses of bone, 

 having the character of enamel, sometimes of considerable size ; 

 but in other cases, being merely small points, as in the rings of 

 Rays, and the shagrined integuments of Sharks. 



2. Ganoides, characterized by the angular form of the scales, 

 which are composed of two substances, namely, a horny material, 

 and a superposed layer of enamel. Many families, both recent and 

 fossil, as the Sclerodermata, Gymnodonta, Goniodonta, Silures, 

 and Sturiones, are included in this order. 



3. Ctenoides, in which the scales are formed of numerous 

 plates pectinated on their posterior edge ; the pectinations being 

 superposed on each other, so that the margin of the one projects 

 beyond that which is above it ; and thus the whole surface is ren- 

 dered rough to the touch. This structure is particularly remarkable 

 in the Chenodonta and Pleuronectes: and is met with also in the 

 Percoides, Polyacanthes, Scienoides, Sparoides, Scorpionoides, and 

 Aulostomata, which, accordingly are referred to this order, 



4. Cycloides, where the scales are formed of simple plates, with 

 a smooth border ; such as the Scomberoides, Gadoides, Sahnones, 

 ClupecB, and Cyprini. 



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