posterior spine. The lateral margins are smooth anteriorly, but bear on the posterior part irregular serrations, which increase in size posteriorly and along the posterior edge consist characteristically of an alternation of one large serra with three or more minute denticulations. The ventral side of the telson was apparently provided with a less prominent keel, though the evidence on this point is not very conclusive.
Appendages. The epistoma, with one of the marginal shields adhering, has been found in one example [pl. 71, fig. 3]. It is a four-sided plate, bounded by a convex anterior, a deeply emarginate posterior, and two broadly concave lateral margins. It is separated by a suture from the triangular marginal shield. Its sculpturing, consisting of crescentic scales with the convexity turned forward, is distinctly seen and verifies by its direction Schmidt's inference that the epistoma, like the marginal shields, originates from extensions of the marginal doublure of the carapace.
The preoral appendages or chelicerae have been found in only one case in connection with the body [pl. 70, fig. 1]. They are in all particulars identical with the more fully known chelicerae of the closely related P. buffaloensis, with the exception of the less prominence of the distal point of the free ramus, thereby indicating a slight approach to P. cobbi. Professor Hall figured a fragment of a free ramus of a chelicera on his plate 80A, figure 6, without determining its relations.
The four pairs of walking legs or endognathites are so thin and slender that they are rarely preserved. They are well seen in Hall's type of P. osborni [pl. 71, fig. 6]. Another still better preserved leg is reproduced in plate 71, figure 8. This resembles an antenna in its whiplike form and exhibits seven segments, the first two of which are short, the other long and tubular, the middle one being the longest, while of the last only a short fragment is seen. The type of P. osborni shows also traces of clublike widened coxal segments. The exact number of these we have been unable to establish. They were relatively longer and more slender than those figured in the restoration of the British forms and of P. osiliensis.
