228 Thirty-fifth Report ok the State Museum. 



AVICULOPECTEK DUPLICATUS. 



Pecten duplicatus, Hall. Geolog. Surv. N. Y. : Rep. Fourth Disk, p. 264. 1843. 

 Aviculopecten duplicatus, Halt.. Pal. N. Y., vol. v, pt. 1. Plates and Explana- 

 tions : PI. 7, figs. 1-7. Jan., 1883. 



Shell ovate-orbicular, sub-equilateral; height usually less than the 

 length; greatest length below the center 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 toward the margins ; crossed by fine, regular, foliate, con- 

 centric expansions of the test. In well-preserved specimens this 

 character of surface gives a decided limate appearance, and somewhat 

 resembles A. rugcestriatus. In weathered specimens this surface 

 character is subdued to a reticulation and crenulation. Near the 

 beaks the shell is nearly smooth ; toward 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 medium example has a height of 27 mm., length 30 mm., and 

 hinge-line 18 mm. 



This species resembles A. rugcestriatus in surface characters, but it 

 is less oblique and proportionally much longer, with beaks more 

 obtuse. The outline of A. cancellatus is more nearly circular, and 

 the direction of the transverse axis is oblique. From A. scabridus it 

 differs in its shorter hinge-line and smaller ears, as well as in the 

 surface markings. 



Formation and localities. In the calcareous shales of the middle 

 Chenfung group, Eockville, Hobbieville and Philipsburg, Alleghany 

 county ; Bast Randolph, Cattaraugus county, N. Y., and Mansfield 

 and Sullivan, Tioga county, Pa. 



