FOSSIL FISHES; CLASSIFICATION. 79 



correspond with the results that have been obtained by the arti- 

 ficial removal of that organ. ( ANIM. PHYSIOL. 465.) The heart 

 also is entirely deficient, and the blood, like that of the Inverte- 

 brate animals, is colourless. 



643. The Geological distribution of this class presents many 

 points of the greatest interest. As might have been anticipated 

 from what is known of the history of the production of the 

 present crust of the globe, we find remains of Fishes in the 

 earliest formations, which distinctly exhibit the action of water, 

 that is, which were deposited as sediments in the bed of the 

 ocean ; and this probably long before any land animals existed 

 upon the surface of the globe. But the Fishes of this early date 

 were, for the most part, formed upon a very different plan from 

 those of the present epoch, so that there are very few of those 

 now existing which bear any close resemblance to them ; whilst, 

 on the other hand, the greater proportion of the species now exist- 

 ing 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 skeleton 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, we find the character of the class gradu- 

 ally 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, that there is a general 

 correspondence between the character of the scales and the in- 

 ternal organisation of the fish ; and he has 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 been accepted by Naturalists, 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 yavoe, splendour. The 

 Fishes of this order are covered by scales or plates, composed 



