Page:Second Geological Survey of Pennsylvania Report of Progress PPP.djvu/21

 towards the dorsum, broad and convex towards the lower portion, and occupying nearly one-half of the entire area. Near the center of the limiting furrow and just anterior, is a strong abruptly elevated node carrying the optic spot, which is usually marked by a slight depression on the summit of the node. There is also a small node in each valve adjacent to the dorsal line, near the posterior apex of the area.

The thoracic portion of the carapace is marked by two large tubercles, a long, sharp, doubly curved ridge, and a row of small nodes along the dorsal line. One of these tubercles is elliptical in outline, oblique, and situated near the middle of the anterior end, back of the optic node. The other elevation is elongate, with its longest diameter at right angles to the hinge, and marked by a small subcentral node. The ridge extends near the lower portion of the valve, concave to the margin along the anterior half of its extent, recurving and nearly following the contour of the lower posterior margin of the valve.

One specimen has been observed which preserves the mandibles in situ, and is represented in Fig. 16 of Plate I. The mandibles are broad and gibbous on the anterior portion becoming narrow below; furnished on their inner edge with a low of denticles; situated near the center of the carapace, and are nearly one-third the length of the valves.

tapering towards the posterior end, composed of six exposed segments, having a total length of one-fourth greater than the length of the carapace. The segments increase in length towards the posterior segment, which has a length equal to two of the anterior segments, and is twice as long as wide; while the anterior somites have a greater width than length. The anterior dorsal margin of each is thickened and arched backwards; the posterior margin is furnished with from four to six strong spines, which project over the articulation of the succeeding segment. On the two anterior joints these spines are much shorter, appearing only as nodes. The ventral side is without spines, but is furnished with broad articulating faces. These characters indicate a great freedom of motion in the parts of the abdomen and account for the many positions in which this member is found.