LOWER IIELDERBERG ROCKS. 
287 
A smaller individual in which the shell is well preserved, showing the stronger and 
intermediate finer radii. The anterior wing is not preserved, and the shell of the 
posterior wing is partially broken away. Some of the concentric striae near the 
base are strongly elevated. 
The interior of the left valve of a specimen of this species, in which the anterior 
wing is imperfect, as well as the cardinal margin of the posterior wing. The 
depressions corresponding to the stronger and finer radii of the exterior are well 
preserved, while the concentric striae upon the posterior wing are equally strong 
with those of the exterior of the shell. 
Fig. 4. The cast of the interior of a smaller specimen, having the posterior side and ex¬ 
tremity of the wing broken off. ( The broader radiating spaces in the figure 
should be represented as the depressed portions, and the narrower as the elevated 
parts.) 
Fig. 5. The right valve, from which the shell has been partially removed. The radii are 
much more equal than on the opposite valve. [ The cardinal margin of the an¬ 
terior wing is improperly represented in the figure.] 
Fig. 6. The interior of a smaller specimen, in which the radii are more equal. 
Fig. 7. A smaller individual (apparently of the same species), having the posterior side and 
base broken off, and the anterior side entire. ( The radii are improperly repre¬ 
sented, as in fig. 4 ). The surface of the specimen is marked by a few strong 
undulations or wrinkles. 
PLATE LIII. 
Fig. 1. A young shell of this species, which preserves the anterior and posterior wings entire, 
with the gibbous umbo and beak elevated above the hinge-line. 
Fig. 4. A specimen of medium size, which is very gibbous above the middle and at the 
umbo. The wing is erroneously represented as too pointed, by leaving out a 
portion which is obscurely visible on the posterior sinuate margin. 
Fig. 6. A similar specimen, in which the body of the shell is a little more oblique than 
usual. The posterior extension of the wing is broken off, so that the margin re¬ 
presented is not the natural one. 
Geological position and locality. In the shaly limestone of the Lower Ilelderberg 
group : Helderberg mountains, Albany county; Becraft’s mountain, near Hudson; 
Schoharie, and other places. 
Fig. 2. 
Fig. 3. 
Plate LII. Fig. 8. 
Shell elongate-ovate, rectangular to the hinge-line; length about once 
and a half the width : basal margin regularly curved, a little more 
produced on the posterior side, gibbous in the middle : anterior wing 
unknown; posterior wing large, triangular. 
Surface marked by eight or nine broad strong ribs, which are depressed 
