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.
The Eurypterids resemble the Xiphosura in many respects. In both groups the prosoma consists of at least six fused segments, and bears two pairs of eyes—one pair simple, the other grouped eyes—on the dorsal surface of the carapace. The number and position of the appendages of the prosoma in Eurypterids agree with those of Limulus. The chelicerae are closely similar in both cases. The coxae of all five pairs of legs in Eurypterids are toothed and function in mastication; similarly in Limulus all are spiny except the coxae of the last pair of legs. In both a similar epicoxite is present on the coxae. The number of joints in the legs is somewhat greater in the Eurypterids than in Limulus, and in the former none of the legs end in chelae, whereas in the latter all the walking legs, except the last, and also the first in the male, may be chelate. The metastoma of Eurypterids differs in being a large unpaired plate, but is represented in Limulus by the pair of relatively small chilaria. On the mesosoma the genital operculum and plate-like appendages with branchial lamellae are similar in both groups, but in the Eurypterids the genital operculum shows a greater development and covers the second segment, which is without plate-like appendages. A striking difference between the two groups is seen in the segments of the mesosoma and metasoma; in Eurypterids these are all free, whilst in Limulus they are fused together, but this difference is bridged over by some of the Palaeozoic Xiphosura (Fig. [159], A) in which those segments are free.
Fig. [166].—Stylonurus lacoanus, Claypole. Upper Devonian, Pennsylvania. Restoration of dorsal surface. Length nearly five feet. (After Beecher.)
The Eurypterids present a striking resemblance to Scorpions. In both groups the segments in the three regions of the body are the same in number, and the appendages of the prosoma also agree in number and position. The pre-oral appendages are chelate in both, but the second pair of appendages are chelate in the Scorpions only. In Eurypterids the coxae of the five pairs of legs are toothed and meet in the middle line, but in the Scorpions the coxae of the last two pairs do not meet; this difference, however, appears to be bridged over in the earliest known Scorpion—Palaeophonus,[[236]] from the Silurian rocks. The Eurypterids are distinguished from the Scorpions by the much greater development of the last pair of legs. The large metastoma of the former is homologous with the sternum of the Scorpion. The genital operculum is much smaller in Scorpions than in Eurypterids, and in this respect the latter agree with Thelyphonus (one of the Pedipalpi) more than with the Scorpions. The pectines are absent in the Eurypterids except in Glyptoscorpius. Instead of the lung-books of the Scorpions the Eurypterids possess branchial lamellae on the plate-like appendages; but this difference between the two groups appears to be bridged over by Palaeophonus, which was marine, and may have possessed branchial lamellae since stigmata seem to be absent.
Glyptoscorpius,[[237]] which is found in the Lower Carboniferous of the south of Scotland, is a form of considerable interest. It is about a foot in length, and agrees in many respects with Eurypterida, but it may be necessary to separate it from that group since it possesses pectines, and the legs end in a double claw; it cannot, however, be regarded as a link between Eurypterids and Scorpions, but must rather be considered as an offshoot from the former, since the latter group was already in existence at a much earlier period.