plain deposits and in deltas" (87, 989). For further references on this subject in the Principles see pp. 377, 425, 945, 950, 1029, 1030.
This gives us, then, the last word on the subject up to the present time. Looking over the opinions which have been recorded in the preceding pages, one is struck with the diversity of conclusions arrived at by our greatest American geologists and by not a few of those of Europe. From 1818, when the first Eurypterus was found, though it was not described as such till 1825, down to the end of the century, it was practically a universal opinion that the eurypterids had been denizens of the sea. The species were described along with marine forms and were considered to have been marine also. The study of the taxonomic position of the Eurypterida, showing always more and more clearly their close relationship to the modern Kingcrab, Limulus, gave an added reason for assuming a marine habitat for the fossil forms. With the beginning of the present century came the awakening of geologists and paleontologists to the fact that perhaps these extinct Merostomata had not always lived in the sea, that they may even never have known marine conditions. The current opinion now is that the eurypterids lived in the sea from Pre-Cambric time through the Ordovicic. During the Siluric they gradually became adapted to brackish and fresh-water conditions, living in estuaries and lagoons in the Devonic and becoming entirely fresh-water habitants in the Mississippic, Carbonic and Permic. Grabau is the only staunch advocate of the non-marine habitat even from the earliest times, though Chamberlin, to be sure, has argued such a possibility, but his discussion is purely philosophical, and while interesting and full of suggestive ideas, it is, nevertheless, unsupported by the evidence necessary for definite proof of his theory.
In attacking this problem the most important thing to determine is whether the Eurypterida began their existence in the sea or in the land waters, and under what conditions they lived in pre-Devonic time, for after that, it is now generally conceded, they lived in terrestrial waters.
