Page:Second Geological Survey of Pennsylvania Report of Progress PPP.djvu/28

14 outwards and backwards over the anterior end. extending in a broad curve along the lower margin. rapidly recurving around the lower posterior portion. and continuing concave and obliquely forward to the extremity of the hinge, mak- in: the hinge-line appear as slightly produced. The cephalic area occupies the anterior third of the valves, and is marked at its base by two broad flattened oval eleva- tions, of which the lower one is somewhat the larger. Optic node prominent. situated above the middle of the area.

Thoraic region convex over the anterior portion, becom- ing liattened on the posterior half. without nodes or ridges.

{{sc|Abdomen having at length of less than half that of the valves, composed of two naked cylindrical segments. The posterior segnient is one-third longer than wide, somewhat larger at its anterior end, Anterior segment partially con- cealed hy the carapace in the specimen described. The margins are slightly thickened.

{{sc|Telson}} consisting of a stout triangular spine flattened on the ventral side. rounded above, and marked by a longitud- inal varina along the middle. Lateral spines longer than the extension of the telson, flattened; margins thickened; marked by two sharp longitudinal ridges on the dorsal face. The inner edges of the movable spines are crenulate for the attachment of setæ

Test thin, not exceeding {{fs|70%|{{over|1|10}}}} mm. in thickness, minutely xvrinkled over the surface ol' the valves. except on their margins and the abdominal segments which are ornarnented with line curving strize or vascular markings.

An entire left valve has a greatest length of 23 mm., width 10 mm., and hinge-line 20 rum. In a specimen preserving a portion of the valves with the abdomen and its apppendages the valves have :1 width of 11 mm., and the length from the distal extremity of the telson to the insertion of the abdomen with the carapace measures 20 mm.

The three specimens of this species observed, present such marked differences from any described form, that it seems advisable to arrange them under a new generic desig- nation. The presence of crenulations along the inner mar-