112 PAUEONTOLOOY OF N£W-yOKK. 



148. 11. LEPT^NA PLANUMBONA ( n. jp.). 



Pl. XXXI. B. Figs. 1 a, b, c, d, e. 



Shell rcsupinatc, robusi, length and breadth as 9 to 11 ; cardinal line straight, suddenly 

 deflected at the extremities, equal to or greater than the width of the shell ; sides a little 

 contracted just below the cardinal extremities, leaving slightly salient angles ; ventral valve 

 flat or slightly depressed near the beak, elevated and very convex in the middle, somewhat 

 abruptly and concentrically deflected towards the margin ; dorsal valve flat on the disc, 

 slightly elevated towards the beak, and deflected to correspond with the otlier valve ; 

 surface marked by radiating strise, every third, fourth or fifth of which is alternated by a 

 stronger one ; entire surface ( in perfect specimens ) marked by fine concentric elevated 

 lines, and a few imbricating lines of growth. 



This is a very neat symmetrical species, presenting very little variation of form. The 

 concentric lines are often obliterated, and the surface shows only the radiating striae. The 

 remarkable flatness, which often amounts almost to a depression of the convex valve near 

 the beak, is a distinguishing character in all the specimens I have examined. The sides 

 of the shell are suddenly constricted just below the extremities of the hinge line, producing 

 small salient ears ; the ventral valve is very convex just forward of the middle, and the 

 opposite valve equally concave. 



Fig. A a 6t, b. Views of the two valves ; the ventral or convex one b showing some strong imbricating 

 lines of growth, while the other valve has but a single one. The beak of the dorsal valve 

 projects beyond the cardinal line of the other valve, as shown in 4 b. 



Fig. 4 c Cardinal view, showing the nearly linear area and closed foramen. There is sometimes a small 

 space for the protrusion of the ligament. 



Fig. 4 d. Interior of the concave valve, showing the visceral impression, which is- nearly circular, 

 depressed in the centre, and longitudinally marked by strong elevated ridges ; it is margined 

 by an elevated border, which proceeds from the two extremities of the foramen, and, curving 

 around, leaves a narrow slit in front There is an elevated line extending from the beak of 

 the shell, through this slit, to the centre of the valve. The margin is marked by strong ribs, 

 which reach from ihe edge to the point of curvature of the shell ; these ribs are produced 

 by the elevated stria; on the outside, which leave a depression upon the inner margin. 



Fig. 4 «. Lateral view of the shell, showing its convexity in the middld, and flattening towards the beak. 



Position and locality. This shell has not been very clearly recognized in New- York, but 

 it is abundant at the West, in the Blue limestone, holding a position equivalent to that of 

 the Trenton limestone, and associated with the preceding species of Leptjena. Among the 

 western localities, I may mention Cincinnati and Oxford (Ohio) , Madison ( Indiana), and 

 Maysville ( Kentucky). (*""<« Collection.) 



