Page:Second Geological Survey of Pennsylvania Report of Progress PPP.djvu/22

 short and broad, flat and quadrangular on the ventral side; triangular and carinated on the dorsal side, and produced into a long, slender spine, having a length about equal to twice that of the posterior segment of the abdomen. To the ventral side of the caudal plate are articulated two movable spines, about equal in length to the spiniform extension of the telson. These spines are grooved along their lateral margins, and marked by a carina on their dorsal face.

Test thin, somewhat thickened on the margins and dorsal line of the carapace, and at the articulations of the abdominal segments; ornamented over the entire surface of the carapace, mandibles, abdomen, and tail with minute granules or pustules, which give a punctate appearance to the whole.

The specimen represented in Fig. 13 of Plate I has a length, exclusive of the tail spines, of 57 m m. The carapace measures 30 m m. in length, 20 m m. in breadth, and 18 m m. along the hinge-line. The segments of the abdomen, beginning with the anterior one, measure respectively 3.5, 3.7. 4, 4, 5, and 8.5 m m. in length,—showing that the posterior segment is more than twice the length of any of the first four segments. The three posterior ones, commencing with the distal segment, have diameters of 5.5, 7, and 8 m m. respectively. The left valve of this species figured in the 16th Rept. N. Y. State Cab. Nat. Hist. has a greatest length of 55 m m., with a breadth of 37 m m. The three abdominal segments figured in the same publication also belonged to a much larger individual than any noted in the present description.

This species differs from E. sublenis,, (Am. Jour. Sci., Vol. XIX, p. 36,) in the more numerous nodes and tubercles of the carapace, the curvature and direction of the ridge along the thoracic portion, and in its more elongated abdominal segments. The same characters serve to distinguish it from E. pustulosa,, (loc. cit.,) with the addition of a marked difference in the surface ornamentation, which in that species is distinctly pustulose. It is readily distinguished from E. socialis, in its larger