of the two halves of this plate being fused. It is slender, being about one fourth as wide as long and does not quite reach the posterior edge of the sternite. The anterior end is slightly constricted where it fuses with the sternite, and the distal is tapering. The male appendage is confined to the operculum. In the material of the collection is a single specimen showing the exterior, the others showing the internal form only. It was evidently small, about one fifth the length of the operculum by which it is surrounded.
The body is covered with comparatively coarse, imbricating crescentic scales, most distinct on the sternites and swimming arms. When the integument of the metastoma and paired appendages are scaled away, there remains a punctate surface. The specimens found show that the size of these animals averages from 20 centimeters to 30 centimeters. A fragment of the third joint of a swimming arm was found, however, which appears to have been part of an individual over 60 centimeters in length.
In the outline, size and proportions of carapace (length : width as 2 : 3) this species resembles E. lacustris more than any other form. Moreover, the abdomen and the legs appear not to have been very different, but the telson is notably longer and more slender than in E. lacustris or in any other of our species of Eurypterus.
Before us are carapaces showing that this species grew still larger than the type specimens indicate. One of these measures 54 × 89 mm, another is 58 mm × circa 98 mm.
One of the original figures [op. cit. pl. 10, fig. 7] shows three ends of appendages with thick, clawlike terminal segments. On the left side, however, two endognathites are shown with thin spinelike terminal segments. As other specimens show that the endognathites did not possess such clawlike segments at their extremities, it is probable that Sarle's interpretation of the specimen is not correct. An inspection of the original shows that the supposed terminal segment on the middle line of the figure is one of the chelicerae, but the two others on the right hand side are oval prominences with a median sulcus but without separation of the test. We surmise, therefore, that they are the upper views of the first segments of endognathites which are displaced. A corrected figure is here given on plate 13, figure 6.
Horizon and locality. Pittsford shale, Pittsford, Monroe co., N. Y.
