TROPIDOCABIS. PPP. 19 



In its geological position this species occurs about two 

 hundred and fifty feet higher in the Chemung 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. 



Formation 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 II, 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 car- 

 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 area 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 ones are simple and sharp. Ab- 

 domen and telson unknown. 



Distinguished from T. m^<?rr?.^^^a by the alternating char- 

 acter of the longitudinal ridges of the carapace. One of 

 the specimens figured shows tlie im])rint of the jilates of 

 an echinoderm which was bedded witli it. 



Formation and locality. — In the sandstones of the AVa- 

 verly group at Warren, Pennsylvania, associated with nu- 

 merous species of brachioi)oda, lamellibranchiata, gastero- 

 poda, and other fossils. 



