8 THE HABITAT OF THE ETJRYPTERIDA 



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 im- 

 portance 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 Eury- 

 terida 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 main- 

 tain 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 fines 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 inter- 

 pretations are offered. These facts include: A, the distribution of all 



