It; P1*P. KK.rour OK ri;<i(;i:Kss. r. k. hkkchem. 



The tluve species in question also illustnUt^ in a very satis- 

 factory manner the successive appeaiance of liiglier types in 

 the same genus. Beginniuu-wirli T. biatrindla, at the base 

 of tlie Cliemung group as exposed at Warren, Pennsylva- 

 nia, we have a species exhibiting but two strong longitu- 

 dinal ridges on each valve. Ascending in the series of rocks 

 at tlie same h)cality, and near the uppei'most beds of the 

 Chemung group, is found T. inierrupta^ the valves of 

 which are each ornamented with from four to five strong 

 cariufB and several shorter intermediate ridges. Three hun- 

 dred and tifty feet higher in the series, in sandstones of the 

 AVaverly group, occur the examples of T. aUernata, which 

 show a still farther increase in the number of longitudinal 

 ridges on the carapace, and with a still greater variation 

 from their alternating size and ornamentation. 



The species of this genus are conspicuously different from 

 EcuixocARis in their more elongate valves, the strong 

 ridges of the carapace, and in the absence of spines or nodes 

 on the abdomen, which is also probably composed of fewer 

 segments. 



Tropidocaris bicarinata, n. sp. 



Plate II, Figs. 3-5. 



Carapace elongate, semi-elliptical, obliquely truncated 

 behind. Dorsal line extending nearly the entire length of 

 the valves. Valves quite convex transversely; length from 

 two and one-half to three times the width. In mature in- 

 dividuals, the greatest width is across the center of the 

 length of the valves. Anterior end pointed. Margins reg- 

 ularly curving from the anterior extremity of the hinge, to 

 the lower posterior end, thence rapidly curving inwards, 

 and extending concave and slighth' oblique, to the ex- 

 tremity of the hinge-line. The margins are strengthened by 

 a thickening of the test, forming a rounded striated ridge, 

 which extends the entire circumference of the valves. 



Cephalic region occupying about one-fourth the area of 

 the carapace, indicated by a slight change in the convexity 

 of the valves and by several low, obscure elevations. The 

 valves are each ornamented with two strong, longitudinal 



