Lamellibranchiata. 241 



Formation and locality. In soft shaly beds of the Corniferous lime- 

 stone, which constitute the upper portion of the group at Lapham's 

 mill, near Victor, Ontario county, N. Y. 



AVICTTLOPECTEtf INSIGSTIS. 



Aviculopecten insignis, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explana- 

 tions : PI. 1, fig. 8; pi. 3, fig. 13. Jan., 1883, 



Shell less than medium size, sub-rhomboidal, moderately oblique ; 

 about one-third longer than high; basal margins rounded ; anterior 

 and posterior margins obtusely angular or abruptly rounded, and 

 extending along the cardinal slopes in a direct line to the beak. 

 Both valves are quite convex; right valve somewhat the less convex. 

 Hinge-line straight, nearly equal to the length of the shell. Beak 

 obtuse-angular, prominent, directed forward, nearly central. Um- 

 bonal region ample, the sides subtending an obtuse angle, and slop- 

 ing more abruptly to the anterior than to the posterior margin. 

 Ears sub-equal, triangular. Anterior ear, concave or flat, defined 

 by a distinct sulcus ; margin deeply concave ; extremity acuminate. 

 Byssal sinus deep and well-marked. Posterior ear flattened, defined 

 by the absence of the stronger radii ; extremity slightly acute ; 

 margin concave. 



Test thin, marked by strong radiating costse, which, in the left 

 valve, are abruptly elevated and increase by interstitial addition ; 

 crossed by infrequent elevated thread-like concentric crenulating 

 striae, which sometimes give a nodose character to the radii, and 

 by finer lines of growth. The right valve is marked by rounded 

 rays arranged in pairs, bifurcating on the umbo, crossed by fine 

 concentric lines of growth. The radiating striae are obscure on the 

 ears, being more strongly marked on the anterior one. 



The largest specimen has a length of 27 mm., height 20 mm., 

 hinge-line 23 mm. A smaller specimen of 18 mm. in length has a 

 hinge-line of precisely the same length. 



This species is less oblique, and the extremity of the posterior ear is 

 less mucronate than in A. bellus and A. ornatus. The surface mark- 

 ings are very similar to the former, but quite distinct from the latter. 



Formations and localities. In limestone above the Marcellus shales 

 at Stafford, G-enesee county; in the softer shales of the Hamilton 

 group at Moscow, Livingston county, and at Hamburgh, Erie county 

 N. Y. 



[Sen. Doc. No. 38.] 31 



