274 PICTURESQUE SKETCHES 



rally have the whole body. This condition is partly 

 owing to the nature of the bed, which must have been 

 originally a fine mud, in which the fish had rotted 

 after death, when the bones separating, and the fins 

 becoming detached, the thin fragile scales, unlike the 

 bony enamelled case of the older fishes, would not 

 afford a coating solid enough to preserve the integrity 

 of the form. Now, in fishes, the bones of the head 

 are so numerous and variable, that they have hardly 

 yet been reduced to a distinct system, and there is 

 thus the greatest possible difficulty in making out the 

 species. Even after the bones of the skull of the 

 more common recent fishes have been prepared as 

 carefully as possible for purposes of comparison, much 

 difficulty remains, in consequence of the ancient fishes 

 not resembling so much those of our own coasts as 

 those of the Indian seas and Southern Ocean, which 

 are necessarily far more rare, especially in a state 

 to admit of the skeleton being reconstructed. It is 

 a remarkable result of the knowledge and careful 

 research of M. Agassiz, that, in spite of this, he has 

 been able to give a tolerably complete sketch of the 

 fossil fishes of the London clay. 



Like the fishes of older formations, the London 

 day species may be distinguished into groups; but, in 

 consequence of the much nearer resemblance to the 

 recent type, these principally belong to the Cycloid 

 and Ctenoid orders of M. Agassiz, very few indeed 

 of the Ganoid order, so characteristic of the older 

 beds, being here found. In fact, while the whole 

 number of the Sheppey species at present known 

 amounts to ninety-two, and only eleven of these are of 

 the more ancient type of Ganoids, thirty-two being 



