FIG. 165.—_Slimonia acuminata_, Salter. Upper Silurian. Restoration of ventral surface, × ⅑. 1–6, Appendages of prosoma; 7, 8, genital operculum; 7–12, mesosoma; 13–18, segments of metasoma; 19, tail-spine; _a_, lateral eye; _b_, metastoma, covering the inner parts of the coxae of the last pair of appendages; _c_, median process of genital operculum; _d_, branchial lamellae seen through the plate-like appendages. (After Laurie.) ] In _Pterygotus_ there is a well-developed epistome (Fig. 164, _a_) between the mouth and the front margin of the carapace, thus occupying the same position as the hypostome of Trilobites (p. 233). The metastoma is always well developed and forms one of the distinguishing features of the Eurypterids; in form it varies from oval in _Eurypterus_, to cordate in _Slimonia_, and lyrate in _Dolichopterus_. The principal modifications seen in the genital operculum are in the form of its median process; in _Slimonia_ this either ends in three sharp points posteriorly (Fig. 165, _c_), or has the form of a truncated cone; its form in _Eurypterus_ has already been described. _Glyptoscorpius_ differs from other Eurypterids in the possession of comb-like organs closely resembling the pectines of Scorpions. _Slimonia_ apparently differs from other genera in that the plate-like appendages on the posterior three segments of the mesosoma do not meet in the middle line (Fig. 165, 10–12). In some forms, such as _Pterygotus_ (Fig. 164), there is a nearly gradual decrease in the width of the segments in passing from the mesosoma to the metasoma; but in some others, which in this respect are less primitive, such as _Slimonia_ (Fig. 165), the posterior five segments of the body (like those of Scorpions) are distinctly narrower and longer than the preceding segments. The long tail-spine of _Eurypterus_ is represented in _Slimonia_ by an oval plate produced into a spine at the end (Fig. 165, 19); whilst in some species of _Pterygotus_ the plate is bi-lobed at the posterior end (Fig. 164, 19). In _Hughmilleria_ the tail-spine is lanceolate.