Lamellibranchiata. 301 



This form differs from A. subdecussata in the greater and nearly 

 equal convexity of the valves, the stronger, rounded rays, and their 

 interruption by the concentric lamellae. 



Formation and localities. In the Hamilton group, at numerous 

 places in Central and Western New York. 



ACTINOPTERJA BOYDI. 



Avicula Boydii, Conrad. Jour. Acad. Nat. Sci., Phila., vol. viii, p. 237, pi. 12, 



fig. 4. 1842 

 Ptennea Boydii, Conrad. S. A. Miller. Cat. Amer. Pal. Foss., p. 201. 1877. 

 Avicula quadrula, Conrad. Jour. Acad. Nat. Sci., Phila. , vol. viii, p. 243, pi. 



13, fig. 5. 1842. 

 Actinopteria Boydii, Hall Pal. N. Y., vol. v, pt. 1. Plates and Explanations : 



PI. 19, figs. 2-24, 26-30; pi. 23, figs. 5, 6 Jan., 1883. 

 Compare Avicula perobliqua, Conrad. Jour. Acad Nat. Sci., Phila., vol. viii, 



p. 235, pi. 12, fig. 1. 1842. 

 Compare A vicula pleuroptera, Conrad. Jour. Acad. Nat. Sci., Phila., vol. viii, 



p. 242, pi. 13, fig. 2. 1842. 



Shell of medium size, rhomboidal ; body ovate, varying in propor- 

 tions, the longitudinal axis at an angle with the hinge-line of from 

 45° to 60° ; length varying from nearly equal to one-fourth greater 

 than the height; margins regularly rounded below, straight and 

 nearly vertical for a short distance in front ; post-basal side ex- 

 tended. Valves convex, the right valve a little less convex than 

 the left. Hinge-line straight from the anterior side of the beak to 

 the posterior extremity. Beak anterior, acute, prominent, inclined 

 forward, rising above the hinge in the left valve. Umbonal region 

 prominent, subtending an acute angle. Ear short, oblique, limited 

 by a deep but not sharply denned sulcus. Wing large, triangular, 

 not distinctly separated from the body of the shell ; margin con- 

 cave ; extremity acute. In the right valve the ear is somewhat more 

 extended, the sulcus not strong, but the byssal sinus is marked; the 

 wing is proportionally larger and usually more acute at the extremity. 

 Test thick; the left valve, in well-preserved specimens, is marked 

 by numerous strong, simple, sharp rays, which are continuous from 

 the umbo to the margin, with rarely intercalated finer rays ; 

 crossed by regular, sharp, elevated, concentric lamellae which (in 

 good specimens) are produced into subtubular, spiniform extensions 

 upon the rays. Lines of growth are seen between the lamellae. On 

 the wing the rays are more subdued, while the concentric lamellae 

 are strong. The ear is marked only by the crowded concentric 

 striae. On the right valve the radii are obsolete on the body and 

 well-marked on the wing, and the lamellose expansions are conspicu- 

 ous. In some casts they appear as undulating elevated lamellae. 



