A fairly well preserved genital appendage of a female Dolichopterus macrochirus [pl. 44] has been observed by us. It is clearly built on the type of that of Eurypterus in showing the two invaginated median lobes with their bifid extremities. The organ was also, as in Eurypterus, of relatively large size.
The genital appendages of Stylonurus are not known.
The, second sternite has also been found to bear a genital appendage in the females of Eurypterus, Hughmilleria and Dolichopterus. In Eurypterus it consists of one short unpaired proximal and two long awl-shaped distal pieces, the whole being covered by the opercular appendage. In Dolichopterus we have seen the impression of two similar slender terminal pieces between the shorter hornlike terminal pieces of the opercular appendage, and therefore feel sure that it possessed an appendage of the second sternite like that of Eurypterus. In Hughmilleria the appendage of the second sternite has been fully described and figured by Sarle [op. cit. pl. 62, fig. 9, 10]. It is small and attenuate with a triangular base.
Probably the second sternite of the female was also furnished with a small appendage in some of the other genera where it has thus far escaped observation.
From present evidence it may be stated as a general proposition that there are two lines of development of the genital appendages of the operculum in the eurypterids, those of the Pterygotus group and those of the Eurypterus group. In the first, the appendage is composed of but one unpaired lobe, which may become more or less elaborated, as in Slimonia. In the second group two unpaired and two paired lobes are developed.
There is little known as to the genital openings in the eurypterids. Woodward has figured [1872, p. 117, fig. 36] small openings on round tubercles on the basal triangular areas of the opercular appendages in Slimonia, which he considers as ovarian openings. Laurie, however, has not mentioned or figured these openings in his investigation of Slimonia [1893] and their presence seems open to doubt, especially since no homologous apertures have been seen in the other genera. Neither have
