THE EURYPTERIDA OF NEW YORK 127 
qualifies his suggestion by admitting that the details of this structure are 
hardly sufficiently known to admit of our attaching very much morpho- 
logical value to it. Hughmilleria exhibits the same type of opercular append- 
age as Pterygotus but is in all other respects a more primitive form than 
the latter. The evidence from both Hughmilleria and the appendages of 
such forms as P. anglicus indicates that the development of the 
organ in question has taken a different course in the Pterygotus and 
Eurypterus branches. - | 
We have observed no facts in either ontogeny or in Strabops that 
would seem to indicate the date of the development of the genital oper- 
culum. Laurie [op. cit. p. 525] has argued that both Limulus and the 
scorpions came off from the eurypterid stem before the great development 
of the genital operculum, because in Limulus an appendage of the second 
abdominal segment is present which has become reduced in the eurypterids 
and in the scorpions the second segment is well developed. The scorpions 
have, however, not been traced farther back than the Siluric, and Limulus 
not even so far. Thus they leave us in doubt as to the date of the suppres- 
sion of the second body segment in the eurypterids, but it is obviously safe 
to assume that it had already taken place in Strabops in view of the typical 
development of all other eurypterid characteristics in that early genus. 
The abdomen of the prototype of the eurypterids was more or less 
terete, contracting gradually and thereby lacking the distinct differentia- 
tion on dorsal view into preabdominal and postabdominal regions by 
any abrupt constriction at the posterior end of the preabdomen. This is 
evidenced by both the adult Strabops and the larvae of the Siluric genera. 
The segments were of nearly equal length, lacking the marked lengthening 
of the caudal part of the body which finds its extreme in Eusarcus. The 
number of the segments was already fixed in the Cambric progenitors, 
Strabops having six tergites and six postabdominal segments. The larvae 
of the Siluric genera have less in the nepionic stage but soon reach the 
full complement, thus attesting in their ontogeny the early fixation of 
the number of segments in their phylogeny. 
