The =metasoma= (Fig. 161, 13–18) consists of six segments which become longer and narrower from before backwards. Each segment is covered by a ring-like sheath or sclerite, and bears no appendages. The posterior end of the last segment is produced into a lobe on each side, and between these lobes the long, narrow tail-spine arises (Fig. 161, 19). The other genera of the Eurypterida do not differ in any important morphological respects from the form just described, All the genera, of which about thirteen have been recognised, are placed in one family. [Illustration: FIG. 164.—_Pterygotus osiliensis_, Schmidt, Upper Silurian, Rootziküll, Oesel. Ventral surface. Reduced. (After Schmidt.) 1–6, Appendages of the prosoma; 7–12, mesosoma; 7, 8, genital operculum; 13–18, metasoma; 19, tail-plate; _a_, epistome; _b_, metastoma; _c_, coxae of sixth pair of appendages. ] =Fam. Eurypteridae.=—The carapace varies somewhat in outline; in _Slimonia_ it is more distinctly quadrate than in _Eurypterus_, whilst in _Pterygotus_ (Fig. 164) and _Hughmilleria_[234] it is semi-ovoid. The lateral eyes are at the margin of the carapace in _Pterygotus_, _Slimonia_ (Fig. 165, _a_), and _Hughmilleria_, but in the other genera, including the earliest form, _Strabops_,[235] they are on the dorsal surface at a greater or less distance from the margin. The pre-oral appendages of _Pterygotus_ (Fig. 164, 1) differ from those of other genera in their much greater length and in the large size of the chelae; they probably consist of a proximal joint and chelae only, although, commonly, they are represented as having a larger number of joints. Unlike _Eurypterus_ and _Pterygotus_, the second pair of appendages in _Slimonia_ (Fig. 165, 2) differ from the third, fourth, and fifth pairs in being distinctly smaller and more slender, and it is probable that they were tactile. Whilst in _Eurypterus_ the fifth pair of appendages are larger than the three preceding pairs, and also differ from them in structure, in the genus _Pterygotus_ (Fig. 164, 5) they agree closely with the second, third, and fourth pairs, and in _Slimonia_ (Fig. 165, 5) they are nearly the same as the third and fourth pairs. The sixth pair of appendages are much larger and more powerful than the fifth pair in nearly all genera; in _Stylonurus_ (Fig. 166), however, the sixth pair are similar to the fifth, both being greatly elongated and slender; also in _Eusarcus_ (_Drepanopterus_) the sixth pair differ less from the preceding pair of appendages than is usually the case. [Illustration: