TROPIDOCARIS, PPP. 15 



gins of the lateral spines of the tail show that it was fur- 

 nished with fimbria similar to those illustrated for Dithyro- 

 caris neptuiii., Hall, in Illustrations of Devorilati Fossils^ 

 explanation of Plates XXIT, XXIII. The other characters 

 are very different, however, from that genus, in 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 Ditliyrocaris are not a mere prolongation of the upper 

 surface 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 group, as exposed at 

 Warren, Pennsylvania. 



TROPIDOCARIS, nov. get^. 

 [r/yoTTf?, carina; z,'ipt^, caris.l 



Carapace bivalve, semi-ovate or semi-elliptical 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 vnlves, or by a differ- 

 ence in convexity from the remainder of thecarapace. Optic 

 node well-marked, and in the species known, is situated on 

 one of the smaller secondary ridges. Rostrum not observed. 



Two segments of the abdomen are known, which are cyl- 

 in-drical and without nodes or spines. The extent of the 

 telson and appendages has not been observed. The species 

 described show a range of from lo to 40 mm. in the length 

 of the carapace. 



This genus differs in tlie form and ornamentation of thecar- 

 apace from Ckkatioc'AUIs. Its relations witii Ki.vmocaius 

 are quite apparent in the outline of the valves and probably 

 equal number of segments in the abdomen. The successive 

 appearance of three distinct spe<!ies having a similarity of 

 ornamentations from widely se})arated liorizons indicates a 

 permanence of cliaracter which is of generic importance. 



