LOWER HELDERBERG ROCKS. 281 



Avicula snbequilatera ( n. s.). 



Plate XLIX A. Tia. 6. 



Shell semielliptical, wider than long ; hinge-line less than the greatest 

 width of the shell below, depressed-convex in the middle ; umbones 

 appressed ; base broadly rounded : anterior wing small, scarcely sepa- 

 rated by a shallow sinus from the body of the shell ; posterior wing 

 large, separated from the body of the shell by a broad shallow sinus, 

 from which the wing bends slightly outwards. 



Surface concentrically striated, without visible radiating striaa. 



The shell is mostly exfoliated from the specimen, and its entire surface characters 

 cannot be determined. 



Fig. 6. The left valve of this species. 



Geological position and locality. In the upper part of the pentamerus limestone 

 of the Lower Helderberg group : Schoharie county. 



Avicula tenuilauiellata (n.s.)- 



Plate LL Fig. 1 & 2. 



Shell orbicularly subovate : left valve scarcely convex ; right valve flat; 

 hinge-line straight, shorter than the greatest width of the shell : 

 anterior wing small, short, acute, separated from the body by a deep 

 narrow sinus, not extending as far forward as the anterior margin of 

 the shell ; posterior wing short, broader than the anterior, acute at the 

 extremity, not extending to the line of the posterior margin of the 

 shell. 



Sdkface marked by a few unequal concentric wrinkles, and by fine, 

 closely arranged, elevated, subimbricating, lamellose striae, which 

 extend over the wings in like manner. Central portion of the shell 

 marked by faint radiating striae. 



[ PALiGONTOLOaT IIL] 86 



