FOSSILS OF OLD RED SANDSTONE. 49 



'i them. Till we are more clear, however, regard- 

 ing the actual affinities of animals, I would 

 i suppose that any judgment as to difficulties in 

 I their grouping in geological formations, or succes- 

 sion in different formations, might well be given 

 somewhat less doginatically than they are by this 

 writer. 



The few fish-remains of the Upper Silurians 

 may be associated with the ample development of 

 this class in the next {Devonian or Old Red Sand- 

 stone) system. They belong to Agassiz's two 

 orders of placoids (these by themselves in the 

 Upper Silurians) and ganoids, the former of 

 which are represented by our sharks and rays, 

 the latter by the bony pike of America and the 

 polypterus of the Nile. Such are the only fishes 

 found till we come up to the chalk formation, 

 when the now predominant orders of cycloids 

 and ctenoids begin.* The Edinburgh reviewer 



* The North British Review presents, as a strong objection, 

 that, " several new ctenoids, which had been found only in the 

 carboniferous system, have been discovered among the fishes 

 brought by Mr. Murchison from the Old Red Sandstone of 

 Russia. Resolved to make out his position, the author asserts," &c. 

 This is an unlucky venture in opposition. The critic evidently 

 meant it to have a very damaging effect, in consideration that the 

 ctenoids are osseous fishes. The fact is, that the fishes brought 



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