518 A. W. GRABAU PALEOZOIC DELTA DEPOSITS OF NORTH AMERICA 



at a correct conclusion regarding the habitat of these Merostomes. Such 

 isolated occurrences, if they represent scattered distribution, are in reality 

 best explained as cases of individual carapaces washed from the land out 

 to sea, and thus they would become evidence for, rather than against, the 

 non-marine habitat of these creatures. The recent discoveries of Eu- 

 rypterid remains in the basal Siluric beds of the Swatara Gap region in 

 Pennsylvania is of great significance, in view of the fact that this period, 

 as already set. forth, was one of strong land drainage and abundant wash 

 of clastic sediments by the rivers of the old Appalachian land into the 

 sea. The rivers which were able to erode the old Taconic Mountain folds 

 of eastern Pennsylvania and carry the resulting material into the sea 

 were certainly able to wash out during flood time an abundance of the 

 shed exoskeletons of Merostomes which inhabited their quieter reaches. 



The most significant occurrence of Eurypterids in the Siluric of North 

 America is in the Pittsford shales and the Shawangunk conglomerate of 

 Salina age and in the Bertie water-lime of Monroan age. 



The Pittsford shales have already been cited as deposited during a 

 period of change from open sea, in which limestones and dolomites were 

 forming, to conditions favoring the deposition of red shales. The occur- 

 rence of fossiliferous dolomites between the shales carrying the Euryp- 

 terids indicates an oscillatory condition, a repeated reassertion of the 

 marine conditions before their final disappearance ; for the occurrence 

 of these abundant remains of Eurypterids at this horizon and not before 

 shows that the marine conditions preceding were unfavorable to the ex- 

 istence of these Merostomes in this region, or at any rate not encourag- 

 ing. That they existed somewhere is shown by the sporadic occurrence 

 of fragments or individuals in the Niagaran limestones of New York and 

 Canada, by the Pennsylvania occurrences, already referred to, and espe- 

 cially by the remarkable examples found in the Kokomo water-limes of 

 Indiana, now correlated with the Lockport horizon.* The latter occur- 

 rence might again be cited as evidence for the marine habitat of these 

 creatures were it not for the fact that they occur in beds of peculiar char- 

 acter, intercalated between beds of normal marine type, and by the fur- 

 ther fact that the remains are those of cast-off exoskeletons floated into 

 this region, as abundantly indicated by the appearance of these fossils. 

 The Kokomo water-lime was a deposit similar in origin to the Bertie, and 

 the discussion of that formation will also apply with slight modifications 

 to the Kokomo occurrence. 



* This corrlation is. however, rejected by Kindle and others. So far as I have seen 

 the marine Kokomo fossils, they are of Monroan types. 



