LAMELLIBRANCHIA TA. 
17 
Aviculopecten duplicates. 
PLATE VII, FIGS. 1-7; and PLATE LXXXI, FIGS. 9, 10. 
Pecten duplicatus, Hall. Geolog. Surv. N. Y. : Rep. Fourth Dist., p. 264. 1843. 
Aviculopecten duplicatus, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations: PI. 7, figs. 1-7. 
Jan., 1883. 
Shell ovate-orbicular, sub-equilateral, height usually less than the length; 
greatest length below the centre of the shell, giving a somewhat truncated 
appearance to the base. 
Valves moderately convex; very similar in dimensions and convexity; 
the central position of the beaks makes the valves nearly equilateral. The 
right valve is marked by a deep byssal sinus. 
Hinge-line straight, central, length from more than one-half to two-thirds 
the length of the shell. Ligamental area narrow. 
Beaks rounded, obtuse, sub-central. Umbo prominent, ample. 
Ears small. Anterior larger and well-defined by the sulcus extending from 
the sinus to the beak. Posterior one small, triangular; margins concave. 
Byssal notch deep and abrupt. 
Test ornamented with regular duplicating rays, which increase in strength 
towards the margins, crossed by fine, regular, foliate, concentric expansions 
of the test. In well-preserved specimens this character of surface gives a 
decided limate appearance, and somewhat resembles A. rugastriatus. In 
weathered specimens this surface character is subdued to a reticulation and 
crenulation. Near the beaks the shell is nearly smooth; towards the 
margins it is sometimes marked by strong, rounded plications. The rays 
and concentric lines are continued over the ears, and the anterior ear usually 
has several stronger plications. 
A right valve, somewhat larger than usual, has a height of 31 mm., length 
35 mm., and hinge-line 20 mm. A medium example has a height of 27 mm., 
length 30 mm., and hinge-line 18 mm. Younger forms vary slightly from 
these proportions ; a small left valve measures 23 mm. in height and length, 
with a hinge-line of 15 mm. 
3 
