ITS ICHTHYOLITES. 113 



when these red sandstone fishes peopled the 

 waters of the Old World. 



Perhaps the most wonderful circumstance con- 

 nected with these ichthyolites is the preserva- 

 tion of their external form. A good specimen, 

 in fact, represents a picture of the animal itself 

 coloured in light red, Indian red, or occasionally 

 varied with Vandyke brown on a grey back- 

 ground, carefully executed, and with many of 

 the details far more minutely finished than if 

 it were an illustration for a modern work on 

 the natural history of fishes. The same pecu- 

 liarity applies to each in a greater or less degree, 

 while among the fossil fish of the chalk and of 

 subsequent formations, instead of this portrait, 

 as it were, of each, the bones alone are usually 

 in a state of preservation, and the ichthyolite is 

 represented by a skeleton. I will endeavour to 

 explain this as familiarly and concisely as I can. 

 All the fish of the old red sandstone epoch were 

 cartilaginous. Those of subsequent formations 

 were principally osseous or bony as are with 

 few exceptions the fish of the present day. The 

 skeleton of the cartilaginous fishes is composed 

 of gristle ; mere animal matter without the addi- 

 tion of the calcareous earth that constitutes bone. 

 It was therefore liable to rapid decay, but as if to 



