qualifies his suggestion by admitting that the details of this structure are hardly sufficiently known to admit of our attaching very much morphological value to it. Hughmilleria exhibits the same type of opercular appendage 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 operculum. 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 suppression 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 differentiation 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.
