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SILURIAN STRATA.

and some of tlie other fossils, particularly the Trilobitcs, whichare rare, are rather obscure, there is no sufficient evidence onwhich it could be safely established ; is it not then probable thatwhilst the schists belong to the lower section of the Silurian system, some portion of the overlying, and even conformablyassociated, sandstones are actually members of the upper sectionsof that system ? But to place the subject in a clear light, it isnecessary to go through the fossil evidence ; and for this pur-pose the class Crustacea will be first selected, as exhibiting themost strongly marked differences in the successive periods.

Crustacea. —Before entering upon the consideration of thisclass of the animal kingdom, the more important in its connexionwith this portion of the stratified crust of the earth as it is thereexhibited in forms at once peculiar and characteristic, it seemsdesirable to give a brief abstract of the various opinions whichhave been published on the nature, analogies, and affinities, ofthose fossil Crustacea which constitute the remarkable familyof Trilohites, or Pabeades of Dalman. And in doing so it may bewell to premise, that however linked together the several generaand species included in that family may appear to be, by thecommon want, in the present state of knowledge respecting them,of either feet or antennae, which are organs of such importancein the study and classification of recent Crustacea , it can scarcelybe doubted, that had they been seen in their living state, suchmarked variations in their external form would have been at-tended by peculiarities both in adjunct organs, and in internalstructure, sufficient to have authorised, not merely a furthergeneric subdivision, but also the formation of more families thanone; and even now, amidst the uncertainty consequent on suchdeficiencies, it may be fairly asserted, that as some of the nume-rous forms exhibited by these fossils approximate to one type,and some to another, they cannot all be brought into connexionwith any one living type. Few, indeed, of fossil bodies havegiven rise to such various and widely differing opinions ; some ofthe earlier writers having even described them as fishes. Lhwyd,for example, referred one of the wide and flattened Asaphs, tothe genus Buglossus or common sole, considering the fossil asthe skeleton of the fish, and quoting Aldrovandus as havingdescribed similar fossils as icthyomorphous bodies. Thus far,therefore, no advance had been made towards a proper under-