On the ventral surface the segments of the mesosoma bear pairs of plate-like appendages, each of which overlaps the one behind like the tiles on a roof. On the posterior (or inner) surfaces of these appendages are found the lamellar branchiae, which are oval in outline (Fig. 165, _d_). Between the two appendages of the first pair is a median process which is genital in function; this pair are larger than the other appendages, and cover both first and second segments, the latter being without any appendages, and they represent the genital operculum of _Limulus_ (Fig. 153, 10). The form of the operculum, more particularly of the median process, differs in the male and female. In that which is believed to be the female (Fig. 162) the median process is long, and extends beyond the posterior margin of the operculum; it is formed of two small five-sided parts at the base which are united at the sides to the two plates of the operculum; behind this is a long, unpaired part, which is pointed in front; this, together with the remaining parts, is not joined to the side-plates of the operculum, so that the latter are here separated from one another. The third part of the median process is shorter than the second, and bears at its end a pair of small pointed and diverging plates, the tips of which reach to the middle of the third plate-like appendages. On the inner side of the operculum there are, in the female, a pair of curved, tubular organs, attached to the anterior end of the median process, where they open, the free ends being closed; the function of these organs is not known, but was probably sexual. In the male (Fig. 163, A, _a_) the median process is formed of two parts only, and is very short, so that the two plates of the operculum unite behind the process. In the female a median process (Fig. 163, B) is also present between the second pair of appendages (belonging to the third segment of the mesosoma); it consists of a basal unpaired part, and of a pair of long pointed pieces which project on to the next segment. Just as in the case of the genital operculum the basal part is united in front to the appendages, the remainder being free, and separating the greater part of the two plate-like appendages. In the complete animal the median process of this segment is covered by the median process of the genital operculum. The remaining appendages of the female, and all the appendages behind the operculum in the male, are without any median process, and the plates of each pair unite by a suture in the middle line. [Illustration: FIG. 163.—_Eurypterus fischeri_, Eichw. Upper Silurian. (After Holm.) =A=, Genital operculum of male; _a_, median process. =B=, Middle part of second appendage of the mesosoma in the female, showing the median process. ]