FISHES ABUNDANT. 



tion as show that the seas of that era had in many places 

 swarmed with such inhabitants. Professor Agassiz, to whom 

 the investigation of the subject has chiefly been committed, 

 ascertained upwards of a hundred species of Devonian fish, 

 to which number it is to be expected that many additions will 

 yet be made. 



In the present era, there are two leading divisions in the 

 class of Fishes, those having an internal skeleton of bone, and 

 those having this skeleton composed of cartilage. The osseous 

 fishes are now many, and the cartilaginous few. In the De- 

 vonian era, it was different, for the cartilaginous fishes then 

 predominated, and they formed a distinguished portion of 

 the marine population, taking a leading part in that duty 

 of keeping down the numbers of the lower animals, which, in 

 the pre-Devonian ages was chiefly executed by the higher 

 mollusca. The Devonian fishes are arranged by M. Agassiz 

 in two orders, with a regard to their external covering, which 

 that naturalist holds to be, in fishes, a reflection of the internal 



FIG. 21. 



B A C 





^ao/v..-',^ 



A, Placoid scale ; B, C, Ganoid scales. 



organization. In the one (jilacoids) it is of irregular enamelled 

 plates, in the other (ganoids), regular enamelled scales, the 

 first being not placed over each other, as scales are, but laid 

 edge to edge, in the manner of a pavement. 



The cejrialaspis has a longish tail-like body, inserted within 

 the cusp of a large crescent-shaped head somewhat like a 

 saddler's cutting-knife. The body is covered with strong plates 

 of bone, enamelled, and the head was protected on the upper 

 side with one large plate, as with a buckler hence the name, 

 implying lucUer-Jiead. A range of small fins conveys the idea 



