UPPER AND LOWER. 117 



name "Cambrian System" was given, merely provisionally, 

 by Sir Roderick, but which Professor Sedgwick still retains 

 as representative of a distinct geologic period ; and it is in 

 these, greatly below the Lower Silurian base-line, as drawn 

 in 1839, that the Bala Limestones occur. The Plynlimmon 

 rocks (a), — a series of conglomerate, grauwacke, and slate 

 beds, several thousand yards in thickness, — intervenebetween 

 the Llandeilo Flags and the Limestones of Bala (6). And, 

 of consequence, the defensive spine of the Onchus, announced 

 in 1847 as detected in these limestones by the geologists of 

 the Government Survey, must have formed part of a fish that 

 perished many ages ere the oldest of the Lower Silurian for- 

 mations began to be deposited.] 



Let us now, after this survey of both the amount of our 

 materials and the order and time of their occurrence, pass 

 on to the question of size, as already stated. Did the ich- 

 thyic remains of the Silurian system, hitherto examined and 

 described, belong to large or to small fishes ? The question 

 cannot be altogether so conclusively answered as in the case 

 of those ganoids of the Lower Old Bed Sandstone whose der- 

 mal skeletons indicate their original dimensions and form. 

 In fishes of the placoid order, such as the sharks and rays, 

 the dermal skeleton is greatly less continuous and persistent 

 than in such ganoids as the Dipterians and Celacanths ; and 

 when their remains occur in the fossil state, we can reason, 

 in most instances, regarding the bulk of the individuals of 

 which they formed part, merely from that of detached teeth 

 or spines, whose proportion to the entire size of the ani- 

 mals that bore them cannot be strictly determined. We 

 can, indeed, do little more than infer, that though a large 

 placoid may have been armed with but small spines or teeth, 

 a small placoid could not have borne very large ones. And 

 to this placoid order all the Silurian fish, from the Aymes- 

 try Limestone to the Cambrian deposits of Bala inclusive, 



