LOWER HELDERBERG ROCKS. 343 



Eatonia singalaris. 



Plate XXXVIII. Fig. 14 - 20. 

 Atrypa singvlans : Vanciem, 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 niuch 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 - IG. 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, 6. Ventral and dorsal views of a cast of ibis 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. 



