In its geological position this species occurs about two hundred and fifty feet higher in the Chetnung series than the beds carrying T. bicarinata, and about three hundred and fifty feet below the beds in the Waverly group. in which are found T alternata. These beds of the Chemung and Waverly groups at Warren are conformable and in uninterrupted succession.
Formatin and locality.—In soft shales belonging to the Chemung group as exposed at an elevation of three hundred feet above the Alleghany river at Warren, Pa.
Tropidocaris Alternata, n. sp.
Plate I1 Figs. 7, 8.
This species is represented in the present collections by two imperfect left valves_which do not admit of an extended description. The carapace is elongate, at least twice as long as wide. and quite convex. The cephalic region is marked by two broad, rounded elevations on the lower anterior end of the valve, and by a prominent node produced by a swell~ ing and elevation of one of the secondary ridges. which cur- ries the optic spot.
Valves furnished with seven or more very prominent, longitudinal ridges, alternating in size, and continuing the Whole length of the thoracic portion of the carapace. On the line separating the cephalic :iron these ridges are more or less interrupted. but again appear and continue appar- ently to the apex of the valve. The stronger ridges are or- namented along their summits with a double row of minute pits, while the intermediate one are simple and sharp. Ab- domen and telson unknown.
Distinguished from T. interrupta by the alternating char- acter of the longditudinal ridges of the the carapace. One of the specimens figured shows the imprint of the plates of an echinoderm which was bedded with it.
Formation and locality.—In the sztndstones of the Wa- verly group at Warren, Pennsylvania, associated with nu- merous species of brachiopda, lamnellibranchiata, gastero- poda, and other fossils.
