LOWER HELDERBERG ROCKS. 277 



Meganibonia obscura ( n. s.). 



Plate XLIX A. Fig. 7. 



Shell subelliptical, elongate, depressed-convex ; posterior slope com- 

 pressed, not alate : beaks prominent, subacute. 



Surface marked by concentric lamellose striae, which are nearly oblite- 

 rated in the specimen. 



The anterior part of the shell, just below the beaks, is broken off, and the cha- 

 racters are all obscure. 



Fig. 7. The right valve of the specimen. 



Geological position and locality. In the shaly limestone of the Lower Helderberg 

 group : Schoharie county. 



Meg:ambonia lata ( n. s.). 



Plate L. Fio. 4. 



Shell broadly ovate or suborbicular, gibbous in the middle and upon the 

 umbo, and gradually sloping to the sides and base ; posterior slope 

 expanded, not alate, compressed ; anterior cardinal extremity abruptly 

 rounded : hinge-line less than the width of the shell below. 



Surface concentrically striated. 



This species bears considerable resemblance to M. orbicularis ; but it is more 

 oblique on its anterior slope, and the posterior basal portion is more expanded. 



Fig. 4. The left valve of this species. 



Geological position and locality. In the shaly limestone of the central part of the 

 Lower Helderberg group : Schoharie county. 



meganiboiiia oblonga ( n. s.). 



Plate L. Fio. 5. 



Shell somewhat elongate-ovate ; hinge-line short, and the shell gradually 

 expanding towards the posterior extremity, gibbous from below the 



