LOWER HELDERBERG ROCKS. 275 



SuEFACE marked by fine concentric striae of growth, with unequal stronger 

 Iamina3 ; 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 coimty; Schoharie; Winfield, • 

 Herkimer county, and other places. 



Megambonia rliomboidea ( u. 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. 



SuKFACE marked by concentric lamellose stria3, 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. 



