186 PISCES. 



stances, these canals are even imbedded in the cranial and facial 

 bones. 



The study of the structure of the scales of Fishes, in reference to 

 the deciphering of the fossil remains of the latter, has recently led 

 M. Agassiz to adopt them as an element of classification, and he finds 

 that the class may be divided in accordance therewith into four 

 great orders. 1st. The Placoidians (**& a broad plate). This or- 

 der contains Fish whose skin is covered irregularly with enamelled 

 scales, often of small size, in which case they give rise to an appear- 

 ance like the shagreen on the skin of the Shark ; or they are of 

 considerable dimensions, and provided with prickles. All the Car- 

 tilaginous Fish of Cuvier, including the various tribes of Sharks 

 and Rays, with the exception of the Sturgeon, are included in this 

 order. 2d. The Ganoidians (yvs, splendor). The scales are here 

 angular, and form horny or bony plates, coated with a thick layer 

 of enamel. To this order belong the Sturgeon, the Bony-Pike 

 (Lepidosteus), and a large number of genera of fossil Fish, termed 

 Sauroid, from their approximation in several respects to Reptiles. 

 3d. Ctnenoidians (*'?, a comb). The Ctenoid Fish are covered by 

 hard scales, jagged on the outer edge like the teeth of a comb, and 

 devoid of enamel. The Perch and many other Osseous Fish will 

 serve as examples. 4th. Cycloidians (^wAc^a circle). The Fish of 

 this last order have their scales soft and flexible, with simple 

 rounded margins and a variety of linear markings upon the upper 

 surface. The Carp, Herring, Salmon, and many other Fish, are 

 comprised under this order, which, with the former, includes almost 

 the whole number of existing species. 



The above divisions, in their general application, are correct, but 

 are not always abruptly defined in nearly approximated genera. In 

 many cases, e. g., in Pelamys sarda, two kinds of scales occur upon 

 the same Fish ; most of them being round and with the margins en- 

 tire (cycloid), while around the pectoral fins the scales present a 

 jagged border (ctenoid). In other cases, in one and the same fam- 

 ily of genera, or even among the species of a single genus, some 

 have ctenoid, others, on the contrary, cycloid scales ; as is exempli- 

 fied in the family of Clupeae, Cyprinodontes, Gobiodae, and others. 



The rays supporting the fins belong also to the tegumentary 

 structures. They consist either, as in the dorsal fin of many Fish 

 e. g., the Perch and Bream, of stiff, pointed, inarticulate horny 

 spines (radii spinosi), or they are soft, divided at the apex, and ar- 



