LOWER HELDERBERG ROCKS. 
177 
however, are. referred to that name, that the true 0. 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. 
There is likewise a closely allied, if not identical form, in the limestones of the 
Upper Helderberg group. 
The casts of 0. tulliensis differ from the present species more conspicuously in 
the parallel direction of the vascular impressions below the muscular imprints of 
the dorsal valve. 
Fig. 2 a , b, c. Dorsal, ventral and profile views of a very symmetrical specimen of medium 
size. 
Fig. 2 d, e . Front and profile views of a larger individual. 
Fig. 2 f. Cardinal view. 
Fig. 2 g, h, i. Dorsal, ventral, and profile views of a large individual. 
Fig. 2 k. Cast of ventral valve. 
Fig. 2 l. Cast of dorsal valve of specimen 2 k. 
Fig. m, n. Front and profile view of the preceding specimen. 
Fig. 2 o, 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. 
©rtliis strophomcnoides (n. s.). 
Plate XIY. Fig. 2 a - l . 
Shell transverse, somewhat 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, 
[ Palaeontology III.] 23 
