Page:Second Geological Survey of Pennsylvania Report of Progress PPP.djvu/32

18 PPP. which, although not found attached to a carapace, is referred to this species on account of the similarity in sur- face ornamentation and from its association with specimens of the carapace of this species.

Formation and locality. In shales at the base of the Chemung group, as exposed at Warren, Pa.

{{c|{{smaller|Plate II, Fig. 6.}}

{{sc|Carapace}} semi-oval in outline, obliquely truncated be- hind. slightly mucronate in front. Dorsal line straight, somewhat shorter than the greatest length of the valves. Valves regularly convex, widest in the middle, length more than twice the greatest breadth. Lower and anterior mar- gins regularly curving to the lower posterior end of the valve, which is abruptly rounded, and from whence the margin is concave to the extremity of the hinge.

Cephalic area marked by a slightly greater convexity than the remainder of the valve, and showing two or three obscure rounded elevations. Optic spots indicated by a swelling or tubercle on one of the short ridges extending from the apex of the valves. Valves ornamented with three strong ridges which extend the entire length, and by three or four similar ridges extending more than one half the length of the valves. Also on the cephalic region there are from four to five smaller carinæ which extend less than half the length of the valve. Abdomen, telson and append- ages unknown.

Test thin, slightly thickened around the margins of the valves.

The right valve described has a greatest length of 23 mm., a width across the middle of about 10 mm., and the dorsal line measures 21 mm.

The carapace of this species somewhat resembles that of T alternata, but in that species the ridges regularly alter- nate in size and the stronger ones are ornamented with a double series of minute pits along their summits. The greater number of carinae on the valves readily distinguishes it from T bicarinala. {{nop}}