Hughmilleria socialis Sarle
Plates 59–63
Hughmilleria socialis Sarle. N. Y. State Palaeontologist Rep't. 1902. p. 1091, pl. 6–9; pl. 10, fig. 1–6, 8, 9; pl. 11–14; pl. 15, fig. 4–6; pl. 24, fig. 1; pl. 25, fig. 1, 3, 4; pl. 26, fig. 3, 5
This is by far the most common eurypterid in the Pittsford shale and is the genotype and best known species. It has been as elaborately described by Sarle as its novelty, its singular features and the excellence of the material deserve. The original description is as follows:
This type is comparatively small, the length of the average individual not exceeding 15 cm. Viewed from either the dorsal or the ventral side, the outline is slenderly lanceolate. In the natural, undistorted condition, the anterior part of the body is flattened, the dorsal and ventral surfaces being slightly convex, while the caudal portion is nearly cylindric.
Cephalothorax. The cephalothorax is semielliptic or subtriangular in outline, the length equaling the breadth at the base, and comprising about one fifth the length of the entire body. Along the gently curving sides and acutely rounded front is a narrow flattened border, striated on the under surface, and not exceeding a fraction of a millimeter in breadth. The dorsal surface is slightly rounded or arched from the side to the center, so that in an undistorted shield 22 mm long, the elevation is about 2 mm. The posterior edge, except for a slight forward curve at the genal angles, is straight. The compound eyes barely break the outline of the shield; they are small, elongate, widest anterior to the middle, the outer side arcuate, the inner formed by three nearly straight edges—a short basal and a little longer anterior, forming slightly rounded obtuse angles with a long inner side. The anterior end of the eye is acute. The length of these organs on the cephalothorax, the dimensions of which were given above, is 5 mm. However, the usual proportion between the length of the shield and eye is as 1 : 4.5. A line drawn connecting the posterior ends of the eyes passes through the center of the shield. The ocelli are situated on a small tumescence cut by this line; they form two minute, ringlike prominences separated by about the length of their diameter.
Abdomen. The abdomen, at the widest point, or between the third and fourth dorsal segments, is a little wider than the base of the cephalothorax. Thus, in one animal measured, these dimensions were respectively 24 mm and 17.5 mm, in a second 23 mm and 17 mm, and in a
