LOWER HELDERBERG ROCKS. 979 



Megaiiiboiiia orata ( n. s.). 



Plate L. Fig. 7. 



Shell nearly symmetrically ovate, the anterior side a little more regular- 

 ly converging than the opposite, becoming very gradually and equally 

 gibbous from the base towards the umbo : beak subacute ; anterior side 

 subauriculate, with a moderately developed muscular scar. 



Surface of cast preserving marks of concentric laminae or wrinkles, and 

 a few stronger undulations. 



This species is somewhat common; but I have been unable to obtain any other 

 than the casts of single valves, the nature of the matrix being unfavorable to the 

 preservation of the shell. In its symmetrically ovate form, and small anterior mus- 

 culor scar, it is quite distinct from the other species described. 



Fig. 7. The east of the right valve. 



Geological position and locality. In the compact beds of the shaly limestone' of the 

 Lower Helderberg group : Helderberg mountains, Albany county. 



A^1cula[?] navifoniiis. 



Plate XLIX A. Fio. 9 a, b, c. 



Avicvla naviformtt : Conbad, Journal of the Academy of Natural Sciences, 1842, Vol. viii, pa. 210, 



pi. 18, f. 11. 



" Lower valve subrhomboidal, slightly ventricose, obscurely radiated. 

 Summit of umbo much above the cardinal line : anterior wing tri- 

 angular ; posterior wing elongated, angulated at the extremity, which 

 extends beyond the line of the posterior extremity of the valve : 

 umbonial slope rounded." 



The figures 9 a and 9 c are from authentic specimens. The posterior extremity is 

 not so much produced as in the figure of Mr. Conrad. The specimens observed are 

 for the most part casts, showing only partially the concentric and obscure radiating 

 stris. All the specimens yet seen are of the left valve, one, only, preserving a small 

 portion of the right wing. Notwithstanding its strongly alate character, I have been 



