FISHES ABUNDANT. 37 



places swarmed with such inhabitants. M. Agassiz, of Xeuf- 

 chatel, to whom the investigation of the subject has chiefly 

 been committed, has ascertained upwards of a hundred species 

 of Devonian fish, to which number it is to be expected that 

 many additions will yet be made. 



The predominating fishes of this system, and the only ones 

 which (as far as fossils show) existed for some ages, 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 organization. Both orders, it is 

 to be remarked at the very first, are manifestly of an inferior 

 character to the two other orders which afterwards came into 

 existence, and still are the principal fishes of our seas, these 

 being covered by true scales, and respectively named ctenoid 

 and cycloid, from the forms of that part of their organization. 

 The two orders of early fish are covered with integuments 

 considerably different in character ; the one (placoids) with 

 irregular enamelled plates, the other (ganoids} with regular 

 enamelled scales, the first being not placed over each other, 

 as scales are, but laid edge to edge, in the manner of a pave- 

 ment. These characters, according to M. Agassiz, were 

 accompanied by a rudimentary or cartilaginous skeleton, 

 while the ctenoids and cycloids possess an osseous struc- 

 ture. 



The cephalaspis has a longish tail-like body inserted 

 within the cusp of a large crescent-shaped head, some- 

 what 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 buckler-head. A range 

 of small fins conveys the idea of its having been as weak in 

 motion as it is strong in structure. In the coccosteus, the 

 outline of the body is of the form of a short thick coffin, 

 rounded, covered with strong bony plates, and terminating 

 in a long tail, which seems to have been the sole organ of 

 motion. While the tail establishes this creature among the 

 vertebrata and the fishes, its teeth, chiselled, as it were, out 

 of the solid bone of the jaw, like the nippers of a lobster, 



