LOWER HELDERBERG ROCKS. 167 



Oitliis eniinens (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. 



SuKFACE finely striated, with frequent bifurcations ; the striae curving 

 upwards to the hinge margin. 



This species, without careful observation, may be confounded with O..ohlata^ 

 which in its 3"oung 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 strise 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 towards 

 the margin. 



This is an extremely rare species. 



Fig. 7 - 10. Young individuals. 

 Fig. 11 & 14 arc referred with doubt 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-marked and characteristic forms of this species- 



Geological position and locality. In the shaly limestone of the Lower Helderberg 

 group : Helderberg mountains, Schoharie and Carlisle. 



