there resembling a pendulum. Another, supposedly the male, appendage has been figured by Woodward [1869, p. 61] and by Laurie [1893, pl. 2, fig. 13]. This is hastate at its proximal end, and sharply pointed at its distal extremity in Woodward's figure while Laurie represents it as a short, blunt process, but Laurie's figure suggests that it was taken from a fragmentary specimen. Woodward and Laurie also figure the pair of triangular areas in front of the process. Schmidt mentions only faint traces of the sutures in P. osiliensis [op. cit. p. 78] and our material has not shown them at all. As they are also absent in Hughmilleria, they are probably a new acquisition in some species of Pterygotus.
In Slimonia the appendages have retained fundamentally the same structure as in Pterygotus, but they show greater elaboration. The frontal
Figure 21 Slimonia acuminata (Salter). Opercular appendages. (From Woodward)
paired triangular areas are distinctly set off. That form of the median lobe which is considered as belonging to the female [see Woodward, 1872, p. 116, fig. 35; Laurie, 1893, p. 513] terminates in three sharp points at its free end, while the other or male form terminates in a more or less truncated cone. This shows two or three deep transverse furrows, which Laurie thinks due to its having been eversible [op. cit. pl. 2, fig. 8].
In Eusarcus the genital appendages are still incompletely known. We have seen only fragments of the female appendage [pl. 33, fig. 3; text fig. 55] which exhibit the triangular areas, the hastate basal portion of the first lobe, and a faint impression of the second lobe. Indications of the interior tubular appendages have also been seen. According to this evidence the whole organ is a simpler expression of that in Eurypterus.
