66 CLASSIFICATION OF FOSSIL FISH. 



the species now existing had no representatives among those, 

 which inhabited the primeval ocean. Many of the latter are 

 known to us only by their scaly coverings, which are frequently 

 preserved with the greatest perfection, when the internal skele-^ 

 ton has disappeared, the scales having the hardness of bone or 

 even of enamel, whilst the skeleton was cartilaginous. As we 

 ascend towards the newer formations, which are nearer the 

 surface of the earth, we find the character of the class gradually 

 changing, the forms, which were predominant in the older 

 rocks, disappearing one after another, and being replaced by 

 others, which bear more resemblance to those now existing. It 

 has been discovered by Professor Agassiz, who has devoted a 

 large part of his life to the study of Fossil Fish, that there is a 

 constant correspondence between the character of the scales and 

 the internal organisation of the fish ; and he lias proposed that 

 the arrangement of the class shall be founded in the first instance 

 upon the form and structure of the Scales. This classification 

 has not yet been received amongst Naturalists, as superseding 

 that of Cuvier ; but the advantages which it presents, in regard 

 to the study of Fossil Fishes, give it a claim to our attention. 

 According to Professor Agassiz, all Fishes may be arranged 

 under the four following groups : 



I. GANOIDIANS ; from the Greek yavos, splendour. The 

 Fishes of this order are covered by angular scales, composed 

 internally of bone, and coated with enamel. The scales are 

 regularly arranged, and entirely cover the skin with an almost 

 impenetrable armour. This order includes the Sauroid fish 

 ( 588), of which the Sturgeons and the Lepidosteus are the 

 only existing representatives ; together with many other peculiar 

 forms, to which we have nothing at all analogous among the 

 Fishes now existing. 



II. PLACOIDIANS ; from the Greek 7rAa, a broad plate. 

 This order contains Fish whose skin is covered irregularly with 

 plates of enamel, often of considerable dimensions, but sometimes 

 reduced to small points, like the shagreen on the skin of the 

 Shark, and the prickly tubercles of the Ray. Among existing 

 fish, this order comprehends only the Sharks and Rays and their 



