2 22 THE HABITAT OF THE EURYPTERIDA 



nite conclusion to be drawn, but I think that such a geographical de- 

 velopment and confinement more satisfactorily accounts for the facts 

 which are known than any others. We are not obliged to believe 

 that Dolichopterus always lived in the rivers of Appalachia; the facts 

 of distribution and relationship could be accounted for otherwise; 

 but this belief requires fewer special conditions than the assumption 

 of very early dispersal by rivers on the two continents, while a marine 

 habitat is entirely out of the question. One of the strongest reasons 

 for my conclusion that Dolichopterus was restricted to Appalachia 

 lies in the evidence offered by the origin of the sediments. In the 

 study of any problem if the lithogenesis of the formations concerned 

 points overwhelmingly to one and only one history for those forma- 

 tions, then slight palaeontological incongruities should not be ac- 

 cepted as vitiating the history pointed by the facts of lithogenesis; 

 the apparent incongruities can generally be turned into confirmatory 

 bits of evidence if a broad enough knowledge and a scientifically 

 guided imagination can be brought into play. Thus, when the nature 

 of the outcrops, the lithological characteristics of the rocks, and, most 

 important of all, the consideration of possible sources of supply for 

 material, all point to the continent of Appalachia as the region whence 

 the Normanskill, Schenectady, and Shawangunk deposits must have 

 come, while these same considerations point just as conclusively to 

 Atlantica for the Bertie deposits, then, if a fragment of a eurypterid 

 in the Schenectady shales shows a faint similarity to a form in the 

 Bertie, and if half a dozen specimens in the Bertie waterlime bear a 

 slight or even pronounced resemblance to species in the Shawangunk, 

 we must attempt to visualize the conditions obtaining on the North 

 American continent during the early Palaeozoic and we must seek 

 the most rational explanation, the one most in accord with our knowl- 

 edge of the laws operating at present, to account for these seeming 

 anomalies. And we should never forget that the geological record 

 has revealed but a few specimens of most species of eurypterids, and 

 that sometimes even a genus is described from a single individual, 

 and that when a writer describes a new species he compares it with the 

 ones already known, drawing analogies where he can; but species 

 which may seem to be very much alike when one has, say, a single 

 member, a carapace, or a claw, of each to compare, might, if a large 

 quantity of perfect material were available, be discovered to be so 

 different that kinship would be found to be entirely lacking where 

 formerly it had been confidently pointed out. 



