ORISKANY SANDSTONE. 
411 
§trophodonta iiiagiiiventra* 
Plate XCII. Fig. 2 a b'c, & 3; and Plate XCY. Fig. 9. 
Strophodonta magniventra : Hall, Regents’ Report, 1857, p. 54; Palasozoic Fossils, p. 14. 
Shell subseinicircular, varying to longitudinally suboval; length usually 
two-thirds the breadth, sometimes equal or greater. Ventral valve 
convex in the central and umbonial regions, flattened towards the 
extremities : beak slightly incurved ; cardinal border sloping from the 
umbo ; hinge-line crenulated, equal to or greater than the greatest width 
of the shell, sometimes extended into rnucronate points beyond the 
lateral margins. Area of ventral valve broad, slightly curved, distinctly 
marked by vertical striae produced by the prolongations of the hinge : 
foramen closed. 
Surface marked by regular, rounded, slightly elevated, radiating striae i 
interior scarcely granulose, having a plicated flabelliform muscular im¬ 
pression, covering nearly two-thirds of its extent, and varying somewhat 
in its form and proportions. The adductor muscles occupy a narrow 
elongated oval space. 
The area sometimes shows a flat triangular space indicating the foramen, 
but I have no evidence that it is ever free. In a single separate valve, 
there is a strong central process underlying the area in the centre, with 
a cavity on each side for the reception of the bifurcate cardinal process 
of the other valve. 
Dorsal valve [of this species?] deeply concave, radiatingly striated; the 
striae finer than those on the ventral valve. The interior of the valve 
is marked by well-defined muscular areas; and outside of these, the 
vascular and ovarian spaces occupy a large part of the shell, having 
a well-defined semielliptical area of a width little greater than the 
length, and somewhat contracted at the cardinal angles. The margin 
outside of the vascular area is striato-punctate. 
This species has been determined from the strongly marked casts of the ventral 
valve, its usual mode of occurrence in the Oriskany sandstone. I have a single ven- 
