58 VESTIGES OF THE 



to their external covering, which that philosopher holds 

 to bo, in fishes, a reflection of the internal organisation. 

 Both, it is to be remarked at the very first, are mani- 

 festly 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 organisation. The two 

 orders of early fish are covered with integuments con- 

 siderably different in character ; the one {j)lacoids) wdth 

 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 pavement. These characters, according to 

 M. Agassiz, were accompanied by a rudimentary or car- 

 tilaginous skeleton, while the ctenoids and cycloids pos- 

 sess an osseous structure. 



Of certain of the ganoids, it is remarked by every 

 geologist, how much they approximate to the form and 

 armature of the crustaceans, an order of the next lower 

 department of the animal kingdom. 



The ce2)hcdas2ns may be considered as making the 

 smallest advance from the crustacean character ; it very 

 much resembles in form the asaphus of lower formations, 

 having 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 vf as protected on the upper 

 side with one large plate as with a buckler — hence the 

 name, implying huckler-head. A range of small fins con- 

 veys the idea of its having been as weak in motion as it 

 is strong in structure. The coccosteus may be said to 

 mark the next advance to the perfect fish type. The 

 outline of its body is of the form of a sliort thick coffin, 



