LOWER HELDERBERG ROCKS. 
t&7 
Oithis eminens (n. s.). 
' Plate XI. Fig. 7, - 14. 
Shell circularly subquadrate, wider than high. Dorsal valve convex in 
the middle, and depressed almost equally towards the margins : beak 
rising but slightly above the hinge line, with a central depression which 
is lost before reaching the middle of the valve. Ventral valve very 
convex near the beak, flattened or depressed towards the base : beak 
very prominent, projecting much beyond the opposite valve, pointed 
and slightly incurved. Hinge line less than the width of the shell. 
Area large, extending nearly two-thirds the entire width of the shell: 
foramen large and high. 
Surface finely striated, with frequent bifurcations ; the striae curving 
upwards to the hinge margin. 
This species, without careful observation, may be confounded with 0. oblata , 
which in its young and half-grown state has the beak much more prominent than 
in the older forms. The larger area and subangular hinge extremities of the present 
species are distinguishing features. The greatest width is below the centre of the 
shell; and the ventral valve, instead of being concave nearly to the beak, is convex 
from the beak to the centre of the shell, and becomes gradually flattened .towards 
the margin, where it is sometimes depressed, giving a slight sinuosity to the front. 
The strige are coarser than in 0. oblata , and, under a lens, exhibit an irregular al¬ 
ternation of coarse and very fine striae, the latter scarcely increasing in size tow T ards 
the margin. 
This is an extremely rare species. 
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Fig. 7 - 10. Young individuals. 
Fig. 11 & 14 are referred with douht to this species. 
Fig. 12. Three views of the same specimen. 
Fig. 13. Three views of a larger individual. 
The specimens 12 and 13 are well-mai’ked and characteristic forms of this species. 
Geological position and locality. In the shaly limestone of the Lower Helderberg 
group : Helderberg mountains, Schoharie and Carlisle. 
