for implicitly accepting the marine origin of all those rocks for which it has been claimed, nor for believing that all fossils found in the Palaeozoic rocks, with the exception of freshwater molluscs, plants, and insects, are the remains of marine plants or animals. Just as there is a growing tendency at the present time to recognize the importance of the wind and of rivers as agents of transportation and deposition in the past, so there is noticeable an awakening from the old belief that all fluviatile organisms began their life in the sea, and only after countless ages of evolution in that realm, migrated first into brackish water and then into the rivers.
The present paper deals with the habitat of a class of crustaceous animals widespread in the Palaeozoic and confined to it. The Euryterida belong to the subclass of the Merostomata in the class Acerata of the phylum Arthropoda. Their nearest relatives are the limulids and scorpions with which latter group they have been classed by certain authors.
While it is generally accepted that some eurypterids lived in fresh water, the majority of palaeontologists at the present time still maintain that the early periods of the racial history of these organisms were passed in marine waters and that it was only, indeed, after their acme in development had been reached that these merostomes, becoming at first euryhaline, finally forsook the sea altogether and lived in rivers and in brackish water bodies until they became extinct at the end of the Palaeozoic. The evidence set forth in support of this hypothesis is so plausible that many have been led to think that there is a large and convincing array of facts sufficient to furnish an indisputable proof that the eurypterids lived during at least a part of Palaeozoic time in marine waters. It was with the purpose of showing that such a proof was really non-existent, and that the observed facts can also, and perhaps more rationally be accounted for in another way, that the present paper was undertaken. The author proposes to formulate a few of the principles which must be borne in mind in considering such a problem, and to point out the inconsistency in the lines of argument generally given to prove that the eurypterids were originally marine organisms. After a review of all the evidence available, the attempt will be made to judge it impartially and to determine which interpretation is really called for by the facts. The first chapter contains a record of facts, without comment; they are the data from which deductions are later made and of which interpretations are offered. These facts include: A, the distribution of all
