LOWER HELDERBERG ROCKS. 
S4IB 
Eatonia singularis. 
Plate XXXVIII. Fig. 14 - 20. 
\ 
Airy pa singulars : Vanuxem, Geol. Report Third District New-York, 1843, pa. 120, f. 3. 
Shell wider than long, varying in form from ovate to transversely el¬ 
liptical or rhomboidal : hinge line very slightly declining from the 
beaks. Ventral valve depressed convex in the middle towards the beak, 
and concave between the centre and the deflected margins; and below 
the middle, extended into a deep broad sinus, which is prolonged and 
turned upwards in front at right angles to the longitudinal direction of 
the shell : beak small, closely incurved. Dorsal valve convex, some¬ 
times gibbous, and sloping abruptly to the margins; having a strong 
mesial fold beginning above the centre, and produced in a broad flat¬ 
tened and greatly elevated extension. 
Surface marked by fine radiating striae, which, in well-preserved speci¬ 
mens, are crossed by much finer concentric striae : a single central 
one, and sometimes two or three of the striae upon the mesial sinus, are 
much stronger than the others; and there is sometimes an impressed 
line down the centre of the dorsal valve. 
The inner margins of the shell are denticulate, but this character is not shown in 
well-preserved specimens : it is seen in the casts, and upon the edges of the shell 
when worn from the exterior. 
Fig. 14 - 16. Ventral, dorsal, and profile views of specimens of the ordinary size. 
Fig. 17 a, b, c, d. Ventral, front, cardinal, and profile views of larger individuals. • 
Fig. 19 a, b. Ventral and dorsal views of a cast of this species. 
Fig. 20. Enlargement of the surface striae. 
Geological position and locality. In the upper part of the shaly limestone of the 
Lower Helderberg group : Helderberg mountains, and Schoharie. 
