CHAPTER XLVI. 



SHELLS OF THE LOWER SILURIAN ROCKS. 



Shells of the Car ado c Formation, Pis. 19, 20 and 21. — Shells of the Llandeilo 



Flags, PL 22. 



Let us now see to what extent the mollusca of the Lower Silurian are distinguishable 

 from those of the Upper Silurian Rocks. In the first place we may repeat, that al- 

 though a very few species of shells are found nearly throughout the Silurian System 1 , 

 by much the greater number which we are about to examine, are dissimilar from 

 those described in the preceding chapter. Secondly, it may be said, that this inquiry 

 has led to the development of an important modification of the distribution of species 

 during those ancient periods ; — namely, that in these, the lowest rocks of which the 

 fossils have been closely scrutinized, the same species is ascertained to pervade a 

 much greater thickness of strata than in the overlying deposits. For, not only are 

 many of the mollusks of the Caradoc Sandstone common to the Llandeilo Flags, but 

 some of them are also detected in strata of the Cambrian System, far, indeed, 

 beneath the upper limit of that system, as at present assumed. Hence we are entitled 

 to assert, that such shells must have lived through much longer epochs than any species 

 in the younger accumulations ; and further, looking to the smaller number of species 

 which we can detect in the oi'der rocks, we may believe, that new forms were less 

 frequently called into existence than in succeeding ages. 



If, however, it be granted that a good many species of mollusks are common to 

 vast accumulations, embracing the Lower Silurian and Upper Cambrian groups, 

 we are still not at liberty to infer that there is no zoological demarcation between the 

 two systems, and therefore that the Silurian System is without a base. We have al- 

 ready shown, that throughout the overlying geological series, no two great contiguous 

 systems are ever abruptly separated from each other, except in those tracts where the 

 order of deposit has been broken up. 



Thus, passing over more modern analogies, we know that wherever the lower beds 



1 Three or four species of shells range from the Ludlow to the Caradoc formation inclusive, though each 

 species is much more common in one formation than in any other. 



I may, however, here state that Trilobites (crustaceans) more precisely mark the age of each deposit in 

 which they occur than mollusks. (See the ensuing chapter.) 



