Zittel's Handbook and in textbooks, will make this point clear. In the former they are represented as attached to the foremost point of the underside of the cephalothorax; in the latter they appear as inserted just in front of the mouth at the base of the epistoma. Schmidt says regarding this point [p. 73]: "Um auf die Scheerenfühler zurück zu kommen, so hätte ich ihren Ansatzpunkt gern, wie meine Vorgänger, nach vorn an der Unterseite des Kopfes verlegt, aber die eben besprochene 3-theilige Umschlagsplatte liess eine andre Auffassung nicht zu, als ich sie oben auseinandergesetzt. An ihrer Oberflache ist nirgend ein Platz für den Ansatz der Scheerenfühler und zugleich war für diese Umschlagsplatte selbst keine andre Deutung möglich." Laurie has not critically discussed this question in his study of the eurypterids, but only suggests that there was perhaps no properly developed articulation between them and the epistoma because they are always found torn off.
While we are not in a position to offer direct evidence on this problem, or at least only such as is inconclusive, we believe in the correctness of Schmidt's inference for the following reasons. One of our specimens shows that the six appendages of one side radiate from one point, which would mean that the chelicerae were inserted directly in front of the first pair of walking legs. But aside from this observation, the fact that the attachment of the chelicerae, in Slimonia, Eurypterus and Hughmilleria, has since been fully established to be directly in front of the mouth as in Limulus, leaves little doubt that the large pincers of Pterygotus, if they are at all homologous to the minute chelicerae of those three genera, must have had the same place of insertion, viz, at the posterior end of the epistoma. We may add that the homology of the large pincers of Pterygotus with the chelicerae of Limulus, Eurypterus etc. would also suggest as an a priori conclusion, their composition of but three segments and the lack of a distinct articulation with the epistoma.
The general conclusion from the foregoing observations is that the chelicerae exhibit a remarkable identity of structure in all genera despite their great differences in relative size.
