

f 



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l\ 





™ 





v. 





« 









F 



I*. 



hoe* 

 an. 



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i 



h 



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i 



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ire* 



■ I 



Cn. IX.] 



AT SUCCESSIVE GEOLOGICAL PERIODS. 



1 



53 



inferiority of grade for the mollusca of that early period. 



It lias been also remai 



fauna of the primary strata, that while the lamellibranchiate 

 bivalves are comparatively few in number, brachiopods 

 of every variety of form are exceedingly abundant. It can- 

 not be denied that the profusion of these last fixes on this 



a stamp 

 argument 



But if we lay 



mil 



balanced by evidence bearing in an opposite direction, 

 and equally derived from the propc 



number 



the 



mollusca. If the Bra- 



chiopods outnumber the Lamellibranchiata, so, on the other 

 hand, do the Cephalopods outnumber the Gasteropods, espe- 

 cially the highest division of these last, those which 



are 



their congeners. 



animals 

 Ammonite, Nautilus 

 *p, last. heiner molluscs 





higher grade, discharged functions now performed to a great 

 extent by Gasteropods, which are lower in the scale. On the 

 whole, it cannot be said that the successive development, in 



com 



means 



rand branch of 



the animal kingdom which is most largely represented in a 

 fossil state. The variety perhaps of types in the testacea is 



greater now than at any former period, but the rate of 

 advance in organisation has been slow indeed, if the only 

 step realised between Lower Silurian and modern times can be 

 expressed by the passage from a tetrabranchiate to a dibran- 

 chiate cephalopod. According to such a rate of progress, we 

 should require a course of ages anterior to the Silurian epocn 

 as great as that which has since elapsed, in order to bring 



about a 



from 



Fossil Fish. — The failure of the paleontologist to detect a 



— ■ 



may 



le bone of any aquatic animal of the vertebrate cl 

 s older than the Ludlow formation of Murchison, 

 uppermost divisions of the Silurian system, is a i 

 imall weight in favour of progressive development 

 still hope to trace back the memorials of the 



We 



great 



class of fishes to strata of higher antiquity ; but when we 



