Page:The Eurypterida of New York Volume 1.pdf/114

 Joh. Walther [1900], Joseph Barrell [Geol. Jour., 1908] have, in recent years, pointed out the absence of carbonaceous matter in littoral and delta deposits as characteristic of an arid climate.

While thus the salt and gypsum-bearing deposits serve to demonstrate the increase of land-locking and salinity of the lagoon and the subarid to arid climatic conditions, the black and green Pittsford shales indicate that then the aridity had not reached its climax and that the lagoon or estuary still received at certain seasons both clastic sediments and fresh water.

It is therefore hardly necessary to infer that the eurypterids of the Salina period lived in a brine. It is quite possible that, when the lagoon became too saline, they withdrew into the brackish water zone of estuaries or deltas. This inference agrees well with the meagerness of the marine brachiopod and mollusk fauna with which they are associated, the brackish water being still today the least inhabited zone of the hydrosphere on account of its frequent changes in salinity; and it is also in full accord with the occurrence of the eurypterids in the Devonic rocks. There is no longer any doubt that the Stylonurus of the Catskill beds inhabited an estuary with brackish water conditions; and in regard to the Old Red sandstones which are currently considered by the British geologists as fresh-water deposits, Kayser in his excellent textbook has noted [p. 168] that the presence of whole layers of brachiopods and other genuine marine shells in the Old Red sandstone of St Petersburg proves the prevalence, at least temporarily, of brackish water lagoon conditions, arid Clarke has described the lagoons indicated by the Upper Devonic deposits of eastern New York, comparing these with the conditions prevailing now in the bar-locked lagoons of the Prussian Baltic with their shifting of freshwater and brackish faunas. The eurypterid beds of Lanarkshire and the Pentland hills, of Ludlow age, are regarded by British geologists as brackish water deposits, for the reason that they contain eurypterids, phyllocarids, limulids, scorpions and myriapods together with fish and land plants. It therefore seems proper to conclude that the eurypterids in Siluric time