Page:Second Geological Survey of Pennsylvania Report of Progress PPP.djvu/29

Rh gins of the lateral spines of the tail show that it was fur- nished with fimbria, similar to those illustrated for Dithyrocaris neptuni, Hall, in Iillustrations of Devonian Fossils, explanation of Plates XXII, XXIII. The other characters are very different, however, from that genus, i11 the absence of an anterior sinus for the reception of the rostral plate, the want of a strong ridge on the valves, the posterior spiniform projection of the lower posterior end of the valve. and in the number of abdominal segments. The posterior spines of Dithyrocaris are not a mere prolongation of the upper snrface of the valves but are produced by the extension of the infolded thickened rim of the lower side of the valves.

Formation and locality. In the lower arenaceous and micaceous shales of the Chemung gimip, as exposed at Warren, Pennsylvania.

[ζροπις, carrina, χαρις caris.] Carapace bivnlve, semi ovate or semi-ellipitcal in outline, obliquely truncated behind. Dorsal line straight nearly equal to the length of the valves. Valves about twice as long as wide. ornamented with one or more strong longitu- dinal ridges. Cephalic region indicated by broad, rounded elevations on the anterior ends of the valves. or by differ- ence in convexity from the remainder of the carapace. Optic node well-marked, and in the species known. is situated on one of the smaller secondary ridges. Rostrum not observed Two seginents of the abdomen are known, which are cyl- indrical and without nodes or spines. The extent of the telson and appendages has not been observed. The species described show a range of from 15 to 40 nun. in the length of the carapace.

This genus differs in the form and ornamentation of the car- apace from Its relations with are quite apparent in the outline of the valves and probably equal number of segements in the abdomen. The succcessive appearance of three distinct species having a similarity of ornamentations from widely separated horizons indicates a permanence of of character which is of generic importance.