36 GEOLOGICAL MEMOIRS. 



of the whole of the fossil remains of this period, among which 

 Pleurodictyum played as important a part as the Trilobites. Other 

 fossils, belonging to different classes of Mollusca, may be added to 

 the foregoing as aids in establishing the boundary-line between the 

 two successive systems, and of these F. Ad. Roemer's work enume- 

 rates already a considerable number. 



I am far from affirming that this boundary is absolutely true, or 

 that there are no species common to the two systems thus bounded ; 

 indeed I am aware of a considerably large number of Silurian species 

 that reappear in the Devonian System, but they are always very few 

 in comparison with the Devonian forms. 



Generally, however, no such limit can be absolute, since nowadays 

 it would be difficult to carry out as a general principle, that each 

 fauna was destroyed in one and the same instant over the whole 

 earth's surface. Destructive accidents of such a kind, if they have 

 ever happened, could operate only on a limited surface, as for instance 

 in Bohemia : here they destroyed the First and Second Faunas, whilst 

 nothing of the kind is seen in Sweden, although both of these faunas 

 exist there and are easily distinguishable. 



Having thus traced out a sketch of the three great Silurian Faunas, 

 as I regard them, I leave to others, better informed on the subject 

 than myself, the task of subdividing in like manner, if it be possible, 

 the Devonian System. Professor F. Ad. Roemer has already fur- 

 nished an interesting work on this subject, and thereby laid the 

 foundation for further researches, which must indeed prove difficult 

 and wearisome, since much depends upon the authentication of the 

 parallelism and general relations of local stratigraphical groups in 

 districts very far apart. As regards the Silurian System, we can, I 

 think, avoid all such controversy, as we assume, with Murchison, that 

 groups of strata are only local subdivisions, the discrimination of 

 which is dependent on the peculiarities of each district. Thus the 

 presence or absence of limestone is sufficient to give rise to very per- 

 ceptible differences in the vertical distribution of the fossils even in 

 districts lying very near to one another. If we compare widely sepa- 

 rated countries, as Europe and America, the difference of climate alone 

 is sufficient to account either for the total absence of identical species, 

 or for their different position in the vertical series ; although the 

 two groups of strata, conformably to the mutual analogy of all generic 

 and specific forms in other parts of the world, comprise one and the 

 same great fauna. Existing nature affords us in this respect the most 

 convincing proofs. Disregarding the stratigraphical groups and sub- 

 divisions, it appears to me that we can arrive at great results, that 

 have a universality and certainty more advantageous for science, than 

 the partial and more or less uncertain and assailable conclusions to 

 which we may be led by the comparison of the subordinate groups, 

 or by attempting to show their exact contemporaneity by means of 

 single species, the appearance or disappearance of which on the whole 

 earth's surface is in such case assumed to have taken place at the 

 same instant. 



These questions, which I have here only slightly touched upon, are 



