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. [Illustration: FIG. 166.—_Stylonurus lacoanus_, Claypole. Upper Devonian, Pennsylvania. Restoration of dorsal surface. Length nearly five feet. (After Beecher.) ]