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.
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.
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.
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.