UPPER SILURIAN FOSSILS. 



225 



grouj ing of each separate system, and in the passage from 

 one system to another ; and that is true, whatever part of 

 *-he ascending geological series we choose to take between 

 the lowest formations and the highest." As he does not 

 state the nature of the difficulties, I cannot undertake to 

 say what argument or what reconstruction of my system 

 may be necessary to meet them. Till we are more clear, 

 however, regarding the actual affinities of animals, I 

 would suppose that any judgment as to difficulties in their 

 grouping in geological formations, or succession in dif- 

 ferent formations, might well be given somewhat less 

 dogmatically 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 Sandstone) system. They 

 belong to Agassiz's two orders of placoids (these by them- 

 selves 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 

 makes a strong point of the placoid and ganoid orders, as 

 unfavorable to the progressive theory. " Taking into ac- 

 count," he says, "the brain, and the whole nervous, cir- 

 culating, and generative system, the placoids stand at the 

 highest point of a natural ascending scale, and the ganoids 

 are also very highly organized." Of certain families of 

 the first order, found in the Old Red Sandstone of Russia, 

 he says, " Let the reader bear in mind that these fishes 

 are among the very highest types of their class, and that 

 we can reason upon them with certainty, because some 



* The North British Review presents as a strong objection that 

 " several new ctenoids, which h/ad been found only in the carbo- 

 niferous system, have been discovered among the fishes brought 

 by Mr. Murchison from the Old Red Sandstone of Russia. Re- 

 solved 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 home by 

 Mr. Murchison are not of the ctenoid order, but belong to a pla- 

 coidan family called Ctenodus. The mistakes made by this writer, 

 in the geological part of his paper, are of a very grave kind, yet 

 only such as many men of scientific eminence may be expected to 

 make when they venture out of their own peculiar department, 

 and rashly underestimate the strength of the arguments'to which 

 they are opposed. 



18 



