the limbs of Limulus and the eurypterids that are among the strongest proofs of their intimate relationship.
Figure 10 Limulus polyphemus; female, from ventral surface
ab, abdomen; an, anus; ch, chelicera; chi, chilarium; cp, cephalothorax; ol, olfactory organ; op, operculum; sp, spine. (From McMurrich)
The six pairs of limbs are currently divided into the preoral (the first) and postoral (the following five pairs). The preoral limbs are the chelicerae or mandibles, the postoral the walking, and burrowing or swimming legs. Besides these the mouth is surrounded by platelike appendages, functioning as lips. Those are the epistoma, endostoma and metastoma.
a The chelicerae in Limulus are small [see text fig. 10] and consist of three segments, the much compressed but relatively long basal segment and the two forming the pincers or chelae. The basal joint articulates, as Holm has pointed out, with an unpaired, lanceolate plate placed between the coxal segments of the first pair of walking legs.
The preoral appendages of the eurypterids exhibit great differentiation. They are smallest in Eurypterus, where they have been described in detail by Holm and have proved to have almost the exact structure of those in Limulus. We figure here the chelicerae of E. remipes and E. lacustris [plate 7, figure 1, and plate 12, figure 1] which verify Holm's observations. Those of Stylonurus have a like structure and similar relative size as shown by Hall and Clarke [see under S. excelsior]. Eusarcus had similar chelicerae which, how-
