LAMELLIBRANCHIA TA. 
93 
Pterinea flabella. 
PLATE XIV, FIGS. 1-21 ; PLATE XV, FIGS. 1, 4-6, 8-10 ; and PLATE LXXXIII, FIGS. 11, 12. 
Aviaula flabella, Conrad. Jour. Acad. Nat. Sci., Phil., vol. 8, p. 238, pi. 12, fig. 8. 1842. 
“ “ (“) Vanuxem. Geol. Surv. N. Y.: Report Third Dist. 1843. 
Compare Pterinea fasciculata, Goldfuss. Petrefaeta Germanise, p. 137, pi. 120, fig. 5. 1840. 
“ “ “ (“) Sandberger, Verstein, Rhein, Schichtensyst, Nassau, p. 293, pi. 
30, fig. 7. 1856. 
“ “ costulata, F. A. Roemer, Dunker and von Meyer. Palseontographica, T. 1, fig. 2. 
Pterinea flabella, (Conrad) Hall. Pal. N. Y., vol v, pt. 1. Plates and Explanations: PI. 14, figs. 1-21; 
pi. 15, figs. 1, 4-6, 8-10. Jan., 1883. 
Shell large; broad or narrow ovate, oblique, rarely erect; length from two- 
thirds to nearly equal the height; and in some extravagant forms the height 
is about double the length; the greatest length below the middle; anterior 
and basal margins regularly curved; the posterior margin from a little above 
the base, follows a nearly straight line to the beak. 
Left valve more or less convex, often gibbous and arcuate. Right valve 
flat or concave, with a little convexity on the umbo; arcuate, to correspond 
with the curvature of the opposite valve. 
Hinge-line straight, extended upon the posterior side; length greater than 
the length of the valve. 
Beak of left valve acute, rounded, situated near the anterior extremity of 
the hinge, curving forwards over the hinge margin. In rare instances the 
beak does not extend quite as far as the margin of the hinge. Umbo of the 
left valve prominent, gibbous, limited by the sulci of the cardinal expansions, 
and subtending an angle of from 30° to 60°. Beak of the right valve 
depressed, rounded, not rising above the hinge; umbonal region depressed- 
convex, gradually merging into the body of the valve, which is concave below. 
Wing large, triangular, nearly flat, varying in proportions, defined (in the 
left valve) by the post-cardinal slope; margin concave; extremity acute, 
rounded. Ear of left valve a simple rounded convex lobe or auriculation, 
defined by a distinct, broad, rounded sulcus; margin rounded above, sinuate at 
its junction with the valve, forming the byssal sinus. The cardinal expan¬ 
sions of the right valve are similar in dimensions to those of the opposite 
valve; nearly in the same plane with the body of the shell, and defined 
only by the surface striae and the sinus. 
