The telson agrees in its slender, spinelike outline with that of Eurypterus. But while that of Eurypterus is flat on the dorsal side and bears a carina on the ventral side, that of H. socialis is flat on the ventral side and carinate on the dorsal.
Horizon and locality. Pittsford shale at Pittsford, N. Y.
Hughmilleria socialis var. robusta Sarle
Plate 63, figure 16
Hughmilleria socialis var. robusta Sarle. N. Y. State Palaeontologist Rep't. 1902. p. 1097, pl. 21, fig. 1, 2
A few fragments have suggested the presence of a larger variety of socialis in the Pittsford shale on which the following notes were made by Mr Sarle:
What appears to have been a varietal form of Hughmilleria socialis is represented by a nearly entire abdomen, two first ring segments and an imperfect metastoma.
The features which distinguish this form are: its larger size; proportionately much greater breadth; the greater convexity of the dorsal posterior edge of the first ring segment and, in some cases, the division of this edge into two, broad smooth lobes; the more noticeable contraction of the abdomen at the second ring segment; and the more rotund form of the metastoma.
The abdomen found lies in the shale dorsal side up, showing the anterior nine segments well preserved. The second and third ring segments are partially disconnected. The breadth of the preabdomen at the widest point, or between the third and fourth segments, is 51 mm, its length 56 mm, the breadth of the first ring segment is 42 mm, of the second 30 mm. The dorsal posterior edge of the first ring segment is entire and very noticeably convex. In each of the isolated ring segments a broad, deep notch produces a bilobation of this edge. A line of pittings close to this edge shows this feature to be natural. The proportions of the most perfect of these segments are: breadth 43 mm length of the dorsal side 22 mm, length of the ventral 11 mm, length of the postlateral lobes 8 mm. The metastoma associated with one of these segments is apparently of a smaller individual and lacks the anterior notched end. At the widest part it measures 14 mm, and from there to the posterior end, 14 mm.
It was at first thought that the distinctive features of these specimens might be merely old age characters of H. socialis, but larger individuals of that species seem to show the same relative proportions as the
