42 PALAEONTOLOGY OF NEW YORK. 



Interior characters not observed, except an indication of a well-defined 

 ligamentaJ area. 



The specimen is 45 mm. in height, 48 mm. in length, and the hinge-line 

 43 mm. 



This species resembles L. paralklodontus, but diflers in the comparatively 

 larger and more extended posterior ear and fewer and stronger rays, without 

 regular alternation of size. L. orbiculatus is more elongate, with more numerous 

 radii and less defined anterior ear. 



The figure is from a gutta percha cast of a mould of the exterior of the shell. 



Formation and locality. In the limestone of the Upper Helderberg group, 

 Le Roy, Genesee county, N. Y. 



Lyriopecten orbiculatus. 



PLATE IV, FIGS. 3-s ; AMD PLATE I. XXXII, FIG. 3. 



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

 Lyriopecten orbiculatus, Hall. MS., vol. v, pt. 1. 1S77. 



" " " Catalogue Amor. Palaeozoic Fossils, S. A. Muxes. 1S77. 



Pal. N. Y., vol. v, pt. 1: Plates and Explanations: PI. 4. figs. 3-8. 

 Jan.. 1SS3. 

 Not Aviculopecti n orbicularis, McCoy. Carb. Foss. of Ireland. 1N44. 

 orbiculatus, " " " " " 



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

 height equal in small and medium individuals ; in large specimens the height 

 is sometimes one-ninth greater than the longitudinal diameter ; basal and 

 lateral margins full and regularly rounded. 



Left valve moderately convex ; umbonal region ample. Right valve con- 

 cave, 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 binge-line, and to the centre of the valve. 



Anterior ear of left valve, in small shells, comparatively large, triangular, 



