THE OLD RED SANDSTONE. 67 
(see Plate IV., fig. 1,) may be regarded as illustrative of the 
general type. It was one of the first discovered of the Caith- 
ness fishes, and received its name in the days of Cuvier, from 
the osseous character of its scales, ere it was ascertained that 
it had numerous contemporaries, and that to all and each of 
these the same description applied. The scales of the fishes 
of the Lower Old Red Sandstone, like the plates and detached 
prickles of the purely cartilaginous fishes, were composed of 
a bony, not of a horny, substance, and were all coated exter- 
nally with enamel. The circumstance is one of interest. 
Agassiz, in his system of classification, has divided fishes 
into four orders, according to the form of their scales ; and 
his principle of division, though apparently arbitrary and 
trivial, is yet found to separate the class into great natural 
families, distinguished from one another by other and very 
striking peculiarities. One kind of scale, for instance, the 
placoid or broad-plated scale, is found to characterize all the 
cartilaginous fishes of Cuvier except the sturgeon ;— it is 
the characteristic of an otherwise well-marked series, whose 
families are furnished with skeletons composed of mere ani- 
mal matter, and whose gills open to the water by spiracles. 
The fish of another order are covered by ctenoid or comb- 
shaped scales, the posterior margin of each scale being toothed 
somewhat like the edge of a saw or comb; and the order, 
thus distinguished, is found wonderfully to agree with an order 
formed previously on another principle of classification, the 
Acanthopterygii, or thorny-finned order of Cuvier, excluding 
only the smooth-scaled families of this previously formed 
division, and including, in addition to it, the flat fish. A 
third order, the Cycloidean, is marked by simple marginated 
scales, like those of the cod, haddock, whiting, herring, 
salmon, &c.; and this order is found to embrace chiefly the 
