Page:Second Geological Survey of Pennsylvania Report of Progress PPP.djvu/30

16 PPP The three species in question also illustrate in a very satis- satisfactory manner the sucessive appearance of higher types in the same genus. Beginning-with T.bicarinata at the base of the Uheinung group as exposed at Warren. Pennsylva- nia, we have a species exhibiting but two strong longitu- tlinal ridges on each valve. Ascending in the series of rocks at the same locality, and near the uppermost beds of the Chemung group. is found T. interrupta, the valves of whit-h are eavh ornamentetl with front four to live strong rarime and several shorter intermediate ridges. Three hun- dered and fifty feet higher in the series, in sandstones of the Waverly group. occur the examples of T. allernata, which slum' 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 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.

elongate. senii-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 tl1e 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 slightly 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 carpace, 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