LOWER HELDERBERG ROCKS, SS:^ 



with some fragments of the shell itself. The form and surface characters are so 

 similar, and the greater and less convexity of the two valves so nearly corresponding 

 to the usual relations of these parts, that I have inferred these specimens to be of 

 the same species. The small spines, which exist on the upper margin of the anterior 

 wing of the right valve, are not shown in the figure. 



Fig. 3. The interior of the right valve, from which the posterior wing is nearly removed. 

 Fig. 4. The impression or mould of the exterior surface of the left valve of the same species. 



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

 group : Helderberg mountains, Albany county. 



Avicala schohariiB ( n. s.). 



Plate LI. Fta. 5 & 5 a. 



Shell subquadrate, nearly equilateral : left valve gibbous in the middle 

 and flattened towards the umbo ; hinge-line scarcely equalling the 

 greatest width of the shell : anterior and posterior sides nearly equal ; 

 entire basal margin forming a part of a regular elliptic curve : poste- 

 rior wing small ; anterior wing unknown. 



Surface of the body of the shell and of the posterior wing marked by 

 numerous round radiating and dichotomizing striae, and these are 

 crossed by regular subequidistant subimbricating lamellose striae which 

 are less conspicuous than the radiating striae. . ": 



The specimen is marked about halfway from beak to base by a strong concentric 

 ridge, or line of interrupted increase, which shows the form of the shell at that 

 period of its growth : the portion of the shell above this line is quite flat, or a 

 little concave. 



Fig. .5. Left valve of this specici. The line apparently corresponding to the anterior wing in 

 the figure is an accidental ridge in the stone. 



Fig. 5 a. Enlargement of a portion of the surface, to show the round radiating atrias and 

 distant lamellose lines of growth. 



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

 group : Schoharie county. 



