LOWER HELDERBERG ROCKS. 
275 
Surface marked by fine concentric striae of growth, with unequal stronger 
laminae; the character being uniform on the body of the shell, and 
upon the alate portion. 
Fig. 7 a. The left valve of a young shell. 
Fig. 7 b. The left valve of a larger individual. [ The anterior lobe or wing is rarely dis¬ 
cernible in the young shells.] 
Fig. 8. A full-grown? individual preserving in part the anterior and posterior extensions, 
the shell being removed from the central prominent part of the fossil. 
PLATE XLIX A. 
Fig. 8. The left valve of a specimen of the same species. 
Geological position and localities. In the lower and middle portions of the Lower 
Helderberg group : Helderberg mountains, Albany county; Schoharie; Winfield, 
Herkimer county, and other places. 
Megambonia rhomboidea (n. s.). 
Plate XLIX. Fig. 9. 
Shell subrhomboid, expanding towards the posterior side, extremely 
gibbous in the middle, abruptly compressed in front, and concave be¬ 
tween the umbones and the anterior auriculate portion of the shell; 
posterior cardinal slope subalate ( imperfect in the specimen), com¬ 
pressed at the margins : umbones prominent, incurved. 
Surface marked by concentric lamellose striae, with a few radiating lines 
where the shell is partially exfoliated. 
The specimen figured shows a slight inequality in the valves, which does not 
appear to be due to accident. 
Fig. 9. The left valve, imperfect at the posterior side, and also at the anterior side, though 
the latter is not shown in the figure. 
Geological position and locality. In the limestone of the Lower Helderberg group: 
Near Carlisle, Schoharie county. 
