100 PICTURESQUE SKETCHES 



change had taken place in the introduction of a large 

 number of species of this class, which was very imper- 

 fectly represented at first, but which continued im- 

 portant for a very long time, and still forms a group 

 peforming a distinct part in the economy of creation. 



Fragments of placoid fishes, whose remains, al- 

 though consisting only of teeth and bony fins, are 

 thus abundant in species, are in some places very 

 common.' 5 ' 5 ' They were not, like those of the former 

 class (the ganoids), securely encased in enamelled 

 armour, but were covered at intervals with small 

 detached plates, which could scarcely serve the 

 purpose of defence. It is probable that this was 

 little needed, and that the animal depended chiefly 

 on its extreme swiftness of motion both to obtain its 

 prey and escape from its enemies ; while the perfect 

 apparatus of teeth (the commonest fossil remains of 

 these fishes) indicates beyond doubt its ordinary- 

 habits ; and the bony rays (also very frequently met 

 with) attest the provision that was made to enable 

 the animal to turn itself on its back and seize its 

 prey when overtaken, with a rapidity and precision of 

 which we are scarcely able perhaps to form an idea. 



The numerous rays, or bony spines, called Icthy- 

 odorulites,^ so often found fossil in these and newer 

 strata, seem to be identical with the bony spine 

 with which the Port Jackson shark is provided, 



* Among the most remarkable localities are several in the carboniferous 

 limestone of Bristol, and others in the same rock in the neighbourhood of 

 Armagh. 



j* \\BvQ (ichthys}, a fish ; fiopv (doru\ a spear ; Xi0og (lithos), a stone : 

 these fossils being spear-like projections from the back and belly of a 

 shark-like fish, supporting fins, and serving probably also as weapons. 



