Page:Second Geological Survey of Pennsylvania Report of Progress PPP.djvu/27

 Chemung group as exposed at Warren, Pa.; belonging to the upper strata of the series when considered in relation to the entire thickness of the group in New York and Pennsylvania.

Carapace bivalve, dorsal line nearly equaling the length of the valves. Valves elongate, longitudinally sub-quadrangular or sledge-shaped, extending posteriorly slightly beyond the hinge-line. Margins thickened, regularly curving from the anterior extremity of the hinge, making a broad sweep along the lower margin, rapidly curving around the lower posterior end, and extending to the hinge.

Cephalic region marked by a distinct optic node near the anterior end of the valve, behind which are two low, broad elevations. Thoracic portion regularly convex. Rostrum unknown.

Abdomen composed of two naked segments. Telson consisting of a short broad spine, with two strong lateral spines, which are crenulated along their inner margins for the attachment of fimbria.

Test thin and without special ornamentation in the typical species of the genus, which shows only vascular striæ along the margins of the valves, and over the segments of the abdomen.

This genus differs conspicuously from in the form and ornamentation of the valves, and in the number and character of the segments of the abdomen. In the outline of the valves it is very similar to, and is distinguished by the absence of the strong longitudinal ridges presented by the species of that genus.

regularly convex, pod-shaped, dorsal line straight, somewhat shorter than the greatest length of the valves. Valves elongate, longitudinally sub-quadrangular or rhomboidal in outline; lngth equal to more than twice the greatest width. Margins thickened, regularly curving