LOWER HELDERBERG ROCKS. tTT 



however, are referred to that name, that the true O. resupinata seems scarcely to be 

 known. 



Our specimens are more gibbous and more finely striated than the form referred 

 to 0. resupinata by Barrande. The smaller forms figured by De Verneuil ( M. V. 

 K. Geol. Russia, PI. xii, f. 5) bear a very near resemblance to our fossil, while the 

 larger forms have a larger area and more elevated beak : the cast of the ventral 

 valve is likewise quite specifically distinct from ours. The smaller forms of the 

 latter author are still more widely separated from the species under consideration. 

 Tliere is likewise a closely allied, if not identical form, in the limestones of the 

 Upper Helderberg group. 



The casts of O. tulliensis diflfer from the present species more conspicuously in 

 the parallel direction of the vascular impressions below the muscular imprints of 

 the doi-sal valve. 



Fig. 2 a, b, c. Dorsal, ventral and profile -views of a very symmetrical specimen of medium 



size. 

 Fig. 2 d, f. Front and profile views of a larger individual. 

 Fig. 2/. Cardinal view. 



Fig. 2 g, h, t. Dorsal, ventral, and profile views of a large individual. 

 Fig. 2 k. Cast of ventral valve. 

 Fig. 2 /. Cast of dorsal valve of specimen 2 k. 

 Fig. in, n. Front and profile view of the preceding specimen. 

 Fig. 2 0, p. Casts of dorsal valves, showing muscular and vascular impressions. 

 Fig. 2 r. Cast of ventral valve of a full-grown individual. 

 Fig. 2 s, t. Front and profile view of the preceding. 



Geological position and locality. In the Upper Pentamerus limestone of the Lower 

 Helderberg group : Helderberg mountains; Schoharie, Catskill, etc. 



Oithis strophomenoides ( n. 8.). 



Plate XIV. Fio. 2a-/. 



Shell transverse, Bomewhat semioval. Ventral valve flattened convex, 

 with a distinct narrow mesial elevation passing from beak to base : 

 beak scarcely elevated above the hinge line, straight. Dorsal valve more 

 convex than the opposite, most elevated between the middle and the 

 beak, from which a distinct narrow depression gradually expands to- 

 wards the front : beak more . prominent than the opposite, obtuse, 



[ PALiBONTOLOaT III.] 23 



