LOWER HELDERBERG ROCKS. 
1 J5«5 
Ihthis discus (n. s.). 
Plate X A. Fig. 7 - 12. 
Shell circular : valves moderately and nearly equally convex. Dorsal 
valve flattened or slightly depressed in the centre near the beak, the 
depression becoming broader and undefined below the middle of the 
shell. Ventral valve regularly convex, becoming flattened towards the 
lateral and basal margins, sometimes a little elevated in the middle. 
Area narrow : length equal to or greater than half the width of the 
shell. Foramen large, usually filled with the trifid cardinal process. 
Surface finely striated : stride somewhat in fascicles, abruptly bending 
upwards towards the hinge line. Ventral valve with a small vascular 
impression. 
This species has hitherto been confounded with the 0. oblata ; but it differs in 
the larger hinge area, the equal convexity of the valves, the flattening of the dorsal 
valve in the middle, and the absence of a depression in the ventral valve, which 
leaves the margin straight : also the striae appear to be somewhat coarser. 
This latter character, together with the nearly equal valves, sometimes causes 
this species to be mistaken for 0. tubulostriata. 
Fig. 7, 8 a, b. Individuals of the smaller known forms of this species. 
Fig. 9, 10. Individuals of larger size. 
Fig. 11. Interior of the ventral valve. 
Fig. 12. Cast of dorsal valve, probably of this species. 
Geological 'position and locality. In the shaly limestone of the Lower Helderberg 
group : Helderberg mountains; Hudson, Catskill. 
