726 
PALEONTOLOGY: C. SCHUCHERT 
ular MgCl2. Here the electrical conductivity of the solution is some- 
what reduced while the rate of nerve conduction is much augmented. 
This is, of course, a striking instance of Loeb's law of the antagonism 
between a univalent and a bivalent cation; even though the bivalent 
cation in this case is magnesium, well known to be a depressant espe- 
cially for muscular activity in Cassiopea. 
A full report will appear in papers from the Department of Marine 
Biology to be published by the Carnegie Institution of Washington. 
THE EARLIEST FRESH- WATER ARTHROPODS 
By Charles Schuchert 
PEABODY MUSEUM, YALE UNIVERSITY 
Received by the Academy, November 24, 1 9 1 6 
In the year 1900 Prof. T. C. Chamberlin published a prophetic paper 
under the title On the habitat of the early vertebrates? In this paper he 
holds that the problem which he is considering admits of no other than 
hypothetical treatment, and proposes the hypothesis that ''The chor- 
date phylum is ... . essentially from first to last a terrestrial 
race, whose main habitat was the land waters and the land itself, though 
still a race that sent its offshoots down to sea from time to time from 
the mid-Paleozoic onwards " (412). He arrives mainly at this conclu- 
sion on the basis that the fish form among animals could only have 
originated through mechanical genesis in swift streams and under a 
mode of life independent of the bottom. The most essential mechani- 
cal feature of rivers is their flow in a fixed direction, and to this insistent 
physical condition anim.al life had to adapt itself, for ''otherwise the 
animal would be swept out to sea and its race be ended as a stream- 
dweller. It is different with ocean currents, for they return upon them- \ 
selves and an animal may yield to them without losing its marine 
habitat" (406). This hypothesis is applied mainly to the origin of the 
fishes and less insistently to the eurypterids. 
In regard to the origin of the eurypterids Chamberlin states: "From 
the occurrence of eurypterids first in marine beds apparently and later 
in fresh-water deposits it has been inferred that they were originally 
sea-dwellers and later became adapted to land waters, but the meager- 
ness of their marine record on the one hand, and their abundance and 
fine preservation in the fresh-water deposits on the other, give point 
to the question whether their early marine record is anything more 
than the chance deposit of river forms borne out to sea" (403). But 
"it may be equally true that the fish and the eurypterids descended 
