Page:Second Geological Survey of Pennsylvania Report of Progress PPP.djvu/46



Carapace semi-oval, nearly one fourth broader than long; sides oblique, nearly straight; margin indented, distinctly limited. Eyes reniform, prominent, situated a little forward of the centre of the carapace and about midway between a median line and the lateral margins. Between the eyes are two broad rounded longitudinal elevations and below are two similar oblique diverging prominences.

Abdomen increasing in breadth from the carapace to the fourth segment, then gradually decreasing to the seventh segment, which is a little more than three-fourths the width of the fourth; the body narrows abruptly at the eighth segment which is about one-half the width of the fourth. The last segment is longer than wide and about one-half the width of the eighth. The posterior angles of the six anterior somites are acute and slightly produced, the seventh is considerably extended and the five posterior somites are armed with strong angular retral spines. The first segment is anchylosed to the carapace.

The telson is very narrow and attenuate; its length about one-third of the entire animal; extremity acuminate.

There are four simple palpi, of which the fourth pair project beyond the margin of the carapace for a distance equal to the width of the carapace, and expose five joints exclusive of the terminal spine. The first pair expose only the last two joints. The margins of the joints of the palpi are serrated, and the outer posterior angles are produced into spines. The palpi terminate in a long curved spine or free dactylus.

The large swimming feet near the carapace are composed of short, broad joints. The penultimate joint has a length more than twice its width. The palette is elongate elliptical in form, with the anterior terminal margin serrate. A small acute terminal plate is inserted in the apex.

The median appendage of the thoracic plate is