Page:The Habitat of the Eurypterida.djvu/255

Rh break or period of emergence has recently been recognized by Grabau as affecting all countries bordering on the North Atlantic and has been recorded for North America, Scotland, Oesel (by the author), and even in what was formerly supposed to be the continuous section in England (89). There was a widespread diastrophic movement at the end of the Niagaran marking a broad expansion of continental areas during Salina time so that perhaps nowhere are there preserved to us the marine sediments of that period. Certainly the North American eurypterids were cut off from marine routes of migration with which most authors like to provide them, and yet migration seems to have gone on. The Salina in North America was a period of aridity west of the mountain mass of Appalachia, but that chain died out northward and probably merged into the continent of Atlantica, there being no northeast Atlantic sea-lobe at that time. Several possible lines of fluviatile migration were open and nothing is more probable than that emigrants from Appalachian north and northeast flowing rivers should have entered some one of the tributaries of the systems on Atlantica. The exact mode of transit can not be determined, but many routes were open. Indeed, it is possible that migration occurred even in Pittsford time from the rivers of Appalachia into one of the Pre-Bertie rivers which we have seen probably existed in the western New York region even during the Niagaran (p. 113 above). This much we may conclude: There were many routes and possibilities of migration open to eurypterids living in the rivers of Appalachia during the Lower and Middle Siluric, but continuous marine paths to Europe were non-existent. Furthermore, the distinctness of the Ludlow fauna as a whole from any of the faunas of Appalachia, but the close relationship of one species from the former to two from the latter is inexplicable for members of a marine fauna, but normal and expectable for members of fluviatile faunas.

The Old Red Sandstone Fauna. The last of the European formations which is believed to have been derived from the continent of Atlantica is the Old Red sandstone. Most the eurypterids occur in the beds in various localities in Forfarshire. By far the most abundant species is Pterygotus anglicus which finds it nearest relatives in Pterygotus bufaloensis and P. macrophthalmus from the Bertie, and P. osiliensis from the Baltic region. The various points of similarity are so well known that it is not necessary to take them up. Pterygotus minor is a small form found associated with P. anglicus, but it