Page:The Habitat of the Eurypterida.djvu/70



In the first chapter I confined myself to facts which consisted of observations made in the field or the laboratory by students of the rocks and of the faunas. Such facts covered data on the geological and geographical distribution of the eurypterids; in the second chapter I gave a resumé of the opinions which have been held by various writers in regard to the habitat of the eurypterids; the remainder of the paper will be devoted to the contemplation of the recorded facts whose interpretation will be undertaken in the light of principles recognized by the school of philosophical geologists. For this reason, I shall in nearly all cases use the deductive method of inquiry, establishing the general principles which may then be applied to the particular case in hand. It is evident, then, that before we can begin to adduce proofs favoring one mode of life or another for the eurypterids, we must have a good classification of habitats in which each type is clearly defined, and we must determine at the very outset whether there are any criteria which may be recognized in the rocks as absolutely diagnostic of the habitats of the past. In so far as deposits in the sea and on the land have received any consideration at all, distinctions have mainly been drawn on the physical character; of the sediments; but I believe that much more accurate and far reaching results are to be obtained from the study of the fossil faunas. These may be investigated from two points of view, either the chorological, or the bionomic; and besides these, there is yet a third line of approach, namely the geological, in which the physical characters and lithogenesis of the sediments, together with the correlation of synchronous deposits constitute the elements. These three lines of investigation deal with three, for the most part mutually independent groups of facts and I am convinced that any one will yield sufficient evidence to determine the nature of any past habitat. In this chapter I shall deal with the bionomic characteristics of modern habitats, and shall give the criteria for recognizing ancient ones, concluding with the special case of the bionomy of the eurypterids. The following chapter will be devoted to the geological evidence regarding the habitats, while I shall defer until the fifth chapter the chorological evidence which is more conveniently discussed with the geological