LOWER I-IELDERBERG ROCKS, 
28 JS 
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 ITelderberg 
group : Helderberg mountains, Albany county. 
Avicula scliohailse (n. s.). 
Plate LI. Fig. 5 & 5 a. 
Shell subquadrate, nearly equilateral i 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 species. The line appai-ently 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 striae and 
distant lamellose lines of growth. 
Geological position and locality. In the shaly limestone of the Lower Helderberg 
group : Schoharie county. 
