LOWER HELDERBERG ROCKS. 16S 



Orthis 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 : striae 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 dififers 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. 



Jig. 7, 8 a, b. Individuals of tlie smaller known forms of tliis species. 



Fig. 9, 10. Individuals of larger size. 



Fig. 11. Interior of tiie 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. 



