158 NEW YORK STATE MUSEUM 
female genital appendage [text fig. 19]. A like pair of tubes has been found 
by the writers in E. remipes and lacustris [pl. 12, fig. 2]. Finally, 
the delicate membranes of the interior of the ventral side and the oval 
attachment areas of the gill plates were described and reproduced by 
photographs. 
It so happens that E. fischeri, which thus has become known 
down to the least details of its integumental structure, differs very little 
from the genotype of Eurypterus, E. remipes; and both Schmidt 
and Holm have suggested that these with E. lacustris may be 
only geographical varieties of the same species, inasmuch as they appear 
at the same geological horizon. But the question remains open whether 
E. remipes, the genotype by right of priority, is a faithful expres- 
sion of the structure displayed by the genus Eurypterus. To determine 
this point we may briefly survey the phylogenetic relations of the species 
of Eurypterus. 
The American species of Eurypterus readily fall into three subdivi- 
sions, viz, (a) those that group themselves around E. remipes; 
(b) those that vary in different directions from this group but are ap- 
proximately contemporary; and (c) the later Carbonic species. Group 
(a) is represented by E. remipes, E. lacustris and its var. 
pachychirus, E. pittsfordensis, and possibly also E. maria, 
It is Eurypterus sensu stricto. Group (b) is a diffuse association com- 
prising aberrant species E. dekayi, E. microphthalmus, 
E.pustulosus, E.kokomoensis, E. prominens and 
E. boylei. Eurypterus microphthalmus 1s representative 
of a small group, also present in England (E. brewsteri), character- 
ized by its small compound eyes and broad, short cephalothorax. Eury p- 
terus dekayi is probably a branch from E. lacustris with 
a number of characters already suggestive of approach to a phylogerontic 
condition. Eurypterus pustulosus, although attaining immense 
size, is aberrant in the exaggerated development of the tuberculation. 
Eurypterus kokomoensis stands apart in a number of char- 
