316 Notice of the " Silurian System of 



which the segments are only gradually developed. In some 

 higher groups all the parts of the body are at all times re- 

 cognisable, and the change is principally in the form and 

 number of the segments. Such metamorphoses have now 

 been established in sixteen genera and twenty-eight species. 

 M. Barrande thinks that this number may be greatly in- 

 creased by new discoveries, but would not extend it to the 

 whole family of trilobites. Some naturalists have even 

 endeavoured to call in question the fact altogether, though 

 we must think without reason, when we take into account 

 its certainty among the living families of crustaceans. M. 

 Barrande refers such doubters to his collections for proof of 

 its truth ; but as few can take a journey to Bohemia for this 

 purpose, we think the remarkable series minutely engraved 

 in the plates 7, 18, 26, 30, may serve for their conviction, 

 if studied without prejudice. 



The geological distribution of these trilobites along with 

 the other fossils was noticed by us in our former article on 

 the introductory portion of M. Barrande' s work. He now 

 sums up his observations on this and other Silurian regions 

 in the following general propositions. 1st, " In consequence 

 of local conditions, the fossiliferous formations of the Silurian 

 system present in each country a series of distinct stages, 

 each characterized, either by the nature of the rocks which 

 compose it, or by a particular fauna, more or less distinct 

 (tranchee). 2d, These local stages, considered individually 

 do not exhibit in general any complete or constant agree- 

 ment, when we seek to establish a parallel between them, 

 by comparing distant countries with each other. In other 

 words, the local stages of different countries are distinguished 

 from each other, either by the nature of their rocks, or by the 

 zoological composition of their faunas, or by the order of the 

 succession of these. Nevertheless, it cannot be overlooked 

 that there always exist very numerous relations between the 

 animal forms that constitute the mass of these local faunas, 

 even at great geographical distances. 3d, If we group the 

 local stages in each of the Silurian regions according to the 

 sum of the analogies noticed among the fossils of all kinds 

 that they contain, and in particular in accordance with the 



