THE EURYPTERIDA OF NEW YORK 427 
the median nodes are also found there. As we have already intimated 
these features are phylogerontic characters which are more fully developed 
in British Carbonic forms. . 
In the same class of phylogerontic characters belong the exaggerated 
development of the postlateral angles of the epimera of the tergites and 
postabdominal segments into thick, striated falcate spines [see text fig. 
44]. In many specimens they seem to be absent [text fig. 43]. This is 
entirely due to the brittle nature of these defensive organs which easily 
broke away and are frequently found detached. 
A similar explanation may be offered for the apparent spineless char- 
acter of the legs [see text fig. 43], for in reality the spines are much stronger 
and thicker than in most earlier eurypterids; and also mostly broken away 
in the specimens. They were also strongly striated [see text fig. 46]. 
The same phylogerontic tendency to spinosity is also shown in the 
development of the distal fringes of the leg joints into spines [text fig. 46] 
and of the posterior margins of the tergites and the fringe of rather long 
teeth developed on the outside of the palette of the swimming leg [see. 
text fig. 43]. Finally the extreme length and slenderness of the telson- 
_ spine is also a character pointing to the same condition. 
Two large fragments which Hall supposed to be those of ectognaths 
were figured and, in the explanations of his plates, referred to a new spe- 
cies, E. potens. One is apparently a gnathobase and the other a 
portion of a sternite. Both indicate specimens of a size much larger than 
the types of E. mansfieldi with which they are associated, but 
as they exhibit an ornamentation quite as in that species, it is quite prob- 
able that E. mansfieldi grew to corresponding proportions. 
Eurypterus pennsylvanicus C. E. Hall 
E. pennsylvanicus C. E. Hall. Am. Phil. Soc. Proc. 1877. 7: 621 
E. pennsylvanicus James Hall. Sec. Pennsylvania Geol. Sur. Rep’t of Prog- 
ress, PPP. 1884. p. 31, pl. 5, fig. 18 
This species was based on a single small carapace, very similar in its 
characters to that of E. mansfieldi but markedly shorter (length 
to width as 3: 5), and coming, from an arenaceous Carbonic shale at the 
