THE OLD RED SANDSTONE SYSTEM. 



685 



with which other important distinctions are found to" coincide, in the same manner as 

 the varieties in the hair of mammalia, the feathers of birds, and the naked or plated skin 

 of reptiles, are connected with structural and functional differences, and are a natural 

 index to them. The value of this discovery is apparent from the fact, that the scales 

 are precisely those remains of the finny races that are most abundant in a fossil state, 

 so that a single scale becomes a clue to the order and habitat of the animal to which it 

 belonged. The divisions proposed by Agassiz are 



I. SCALES ENAMELLED. 



1. Placoidians from ?rXa, a broad plate. The fishes of this order are irregularly 

 covered with large or small plates of enamel, sometimes reduced to mere points, like the 



shagreen on the skin of the shark, and 

 the prickly tooth-like tubercles of the 

 skin of the ray. It comprehends all the 

 cartilaginous fishes of Cuvier except the 

 sturgeon. 



2. Ganoidians from yavoe, splen- 

 dour, referring to the brilliant surface of 

 their enamel. Angular scales charac- 

 terise the families of this order, composed 

 internally of bone, and externally of 

 enamel, generally bright and smooth. 



The bony pike of the North American lakes and sturgeons are of this order. Upwards 

 of sixty genera have been noticed, of which fifty belong to the fossil kingdom. 



II. SCALES NOT ENAMELLED. 



3. Ctenoidians from Kreig, a comb. These have their scales jagged or pectinated, 

 like the teeth of a comb, on their posterior margin, of which the perch affords a familiar 

 example. 



4. Cycloidians from KVK\OQ, a circle. Fishes of this order have their scales smooth 

 and entire at their posterior margin, composed of horn or bone, of which the herring, 

 salmon, and carp are instances. 



Both osseous and cartilaginous fishes are included in each of these orders ; but only 

 those of the first and second grand divisions, with enamelled scales, Placoidians and 

 Ganoidians, existed in the ancient strata antecedent to the chalk. The third and fourth 

 orders, Ctenoidians and Cycloidians, to which three fourths of the existing species of fishes 

 belong, make their appearance in that formation, when all the preceding fossil genera 

 had become extinct. The following table exhibits the geological distribution of these 

 orders : 



