LOWER HELDERBERG ROCKS. 
28® 
PLATE EH. 
Fig. 9. The left valve of this species, preserving the posterior wing and the greater part of 
the body of the shell, the anterior portion being broken off. 
Fig. 10. The interior of a left valve, which, in the general form and obliquity of the body 
of the shell, corresponds to this species, but the wing is shorter. The abrupt 
separation between the body of the shell and the wing may be due to pressure, 
which has produced a slight folding along that line. 
PLATE LIII. 
Fig. 2. The right valve of a young shell which is imperfect, but, in the obliquity of the body 
and general form, corresponds to this species. It is moderately convex, a little 
gibbous on the umbo, and the surface of the body of the shell shows subdued 
radii, while the wing shows only concentric markings as in A. communis. 
Fig. 3. The left valve of a young shell preserving the surface markings, with radiating striae 
upon the wing, as in the larger specimens. The posterior part of the wing is 
broken off, so that the full extent is not shown in the figure. 
Fig. 5. The interior of the left valve of a shell of this species. In the figure, the sinus on 
the outer magin is too abrupt : a cast from this interior presents precisely the 
characters shown in fig. 9, Plate lii. 
Fig. 7. The left valve of a small specimen of this species : the anterior wing and beak are 
broken off, as well as a part of the posterior wing. 
Fig. 10. A cast of the left valve of this species, from which the shell has been exfoliated 
and the posterior and basal margins broken off. The specimen is a little more 
abruptly convex in the middle and upper part than usual in shells of this species. 
Geological position and locality. In the shaly limestone of the Lower Helderberg 
group : Albany and Schoharie counties; and Litchfield, Herkimer county. 
Ayicula bellula (n. s.). 
Plate LIII. Fig. 8 & 9. 
Shell subrhomboidal, with a rotund base ; length equal to once and a 
half the height; hinge-line a little shorter than the length of the shell: 
left valve convex, or a little gibbous in the middle; right valve nearly 
flat or very slightly convex : anterior margins abruptly rounded ; po¬ 
sterior margin a little produced and rounded, giving a broad regularly 
curving ventral margin : anterior wing small, triangular, acute, pro¬ 
jecting as far as, or a little beyond the margin of the shell, distinctly 
separated by a sinus from the body of the shell; posterior wing 
broader, not conspicuously separated from the shell, nor reaching as 
far as the posterior margin. 
Surface of the left valve marked by strong dichotomizing or somewhat 
[ Paleontology III.] 37 
