LAMELLIBRANCHIATA. 17 



AviCULOPECTEN DUPLICATUS. 

 PLATE VII, KIGS. 1-7; ANli PLATE LXXXI, FIGS. 9, 10. 



Pecten dupHcatus, Hall. Geolog. Siirv. N. Y. : Rep. Fourth Dist., p. 264. 1843. 



Aviculopecteu dupllcatvs. Hall. Pal. N. Y., vol. v, pt. 1. Pliitp.s and Explanations; PI. 7, fig.s. 1-7. 



Jan., 18S8. 



Shell ovate-orbicular, .sub-eciuilateral, 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. rugcRstriatus. 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 

 .85 mm., and hinge-line 20 mm. A medium example has a height of 27 mm., 

 length .SO 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. 



