MANUAL OF NATUKAL HISTOEY. 81 



described, of which some knowledge is required, as 

 they are frequently mentioned in the following classi- 

 fication. Agassiz has considered them of such im- 

 portance that he has founded an arrangement on 

 their differences, whereby fishes are divided into 

 four orders, named respectively Ganoidians, Placoi- 

 dians, Ctenoidians, and Cycloidians. Ganoid scales 

 are so designated from their being covered with 

 bright enamel ; they are mostly thick and angular. 

 Species of this order chiefly belong to extinct forms, 

 the Sturgeons, and a few others, being the only living 

 representatives. Placoid scales are composed of hard 

 osseous matter, sometimes large, at other times re- 

 duced to small points, as when they form the prickly 

 tubercles of the Skates. The third variety, or Ctenoid, 

 derives its title from having the outer or posterior 

 edge toothed like a comb. It is found occasionally 

 in fossil genera, and is abundantly numerous among 

 the Perches and other allied families. The last, or 

 Cycloid, is that which is most characteristic of the 

 fish of the present era, and is composed of smooth 

 rounded layers of horny matter, entire at the mar- 

 gins. 



The external configuration of fishes is extremely 

 diversified. The most usual appearance is that of a 

 cylindrical body, more or less pointed at the extre- 

 mities. Others, as the common Cod, are broader 

 anteriorly, and taper towards the tail. Some are 

 short and broad, as the Chsetodons ; others, as the 

 Eels, long and narrow. Some present a considerable 

 thickness of body, as the Salmons ; while others, like 



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