68 NEW YORK STATE MUSEUM 
and in the remarkable specimen of Eurypterus kokomoensis, 
reproduced in plate 26, figure 2, the branchial plates are distinctly set off 
from a very tenuous brownish film, the integument of the individual, as 
thick, jet-black oval plates with a coarsely granular surface.’ These gill 
plates were here very much smaller than those of the following sternites, 
those of the third and fourth sternites being the largest and those of the 
last pair of sternites again considerably smaller. On the plates of the 
operculum and of the next sternite, projecting thick-walled tubes can be 
seen that perhaps correspond to the trunk veins observed by Holm. 
Metasoma, postabdomen or tail. The tail of the eurypterids con- 
sists of six ringlike segments, which decrease in width and correspondingly 
increase in length in posterior direction. In the primitive genus Strabops 
they merely decrease in width; and in all more primitive forms, as Strabops 
and Hughmilleria, and the simpler species of Eurypterus, the decrease 
in width is very gradual. In others it is abrupt in the first and 
second postabdominal segments. The first postabdominal segment in 
most species is a strongly contracted conical ring. In forms where the 
preabdomen has been excessively broad, the contraction takes place mainly 
in the last two tergites and the first postabdominal segment. In Eusarcus, 
where the contrast between the broad preabdomen and the narrow tail- 
like postabdomen has become extreme, the first caudal segment contracts 
by more than one half, while the following segments have nearly uniform 
width, forming a cylindrical tal. | | 
The segments fit into each other like the joints of a telescope, the 
posterior one always reaching, with its anterior articulating edge, as far 
as the posterior doublure of the anterior segment. They consequently 
articulate in all directions and are therefore found either extended straight 
or curved to either side. An exception to this rule seems to be presented 
by Eusarcus in which the tail is nearly always curved to one side— 
although specimens with straight tails have been observed in the short- 
1The plates are much more distinct on the specimen than the photograph was 
able to bring out, owing to a thin whitish film over them. 
