Lamellibranchiata. 247 



This species resembles L. parallelodontus, but differs in the compara- 

 tively larger and more extended posterior ear and fewer and stronger 

 rays, without regular alternation of size. L. orMculatus is more 

 elongate, with more numerous radii and less defined anterior ear. 



Formation and locality. In the limestone of the Upper Helderberg 

 group, Le Roy, Genesee county, N. Y. 



Lyriopectek orbiculatus. 



Avicula orbiculata, Hall. Geolog. Surv. N. Y.t Rep. Fourth Dist., p. 202. 1843. 

 Lyriopecten orMculatus, Hall. MS., vol. v, pt. 1. 1877. 



" " " S. A. Miller, Cat. Amer. Pal. Foss, Cincinnati, 



1877. 

 " " •' Pal. N. Y., vol. v, pt. 1. Plates and Explana- 



tions : PI. 4, figs. 3-8. Jan., 1883. 

 Not Amculopecten orbicularis, McCoy. Carb. Foss. of Ireland. 1844. 

 " " orbiculatus, " " " " 



Shell large, orbiculate, becoming wider and oblique with age ; length 

 and height equal in small and medium individuals ; in large speci- 

 mens the height is sometimes one-ninth greater than the longitu- 

 dinal diameter; basal and lateral margins full and regularly rounded. 

 Left valve moderately convex ; umbonal region ample. Eight 

 valve concave, flat or slightly convex over the umbonal region. 

 Hinge-line straight, central ; length in young shells equal to four- 

 fifths or five-sixths of the longitudinal diameter ; and in large 

 specimens the length is sometimes not more than one-half the 

 length of the valve. Beak obtuse, slightly oblique to the hinge, not 

 rising above it, situated anterior to the middle of the hinge-line, 

 and to the center of the valve. Anterior ear of left valve, in small • 

 shells, comparatively large, triangular, limits not strongly defined ; 

 margin straight or slightly concave ; extremity acute-angular ; in 

 older shells comparatively narrow, with an obtuse-angular ex- 

 tremity. Anterior ear of the right valve defined by a sulcus, 

 extending to the extremity of the beak ; margin convex ; extremity 

 rounded. Byssal sinus scarcely indicated on the left valve, forming 

 a deep, angular notch in the right valve. Posterior ear large, broad- 

 triangular ; length about two-thirds of the hinge-line ; margin 

 concave ; extremity acute or right-angled in small specimens, obtuse 

 in larger individuals ; limits not defined. 



Test thin, thickness nearly 3 mm., ornamented by about 80 

 rounded radii, with broader concave interspaces, crossed by regular, 

 sharp, elevated, continuous, concentric, foliate expansions of the 

 test, reaching an elevation of about .6 m. above the general sur- 



