LAMELLIBRANCHIATA. II:; 



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 lamella?. 



Formation and localities. In the Hamilton group, at numerous places in 

 Central and Western New York. 



Ai'TINOPTERIA BOYDI. 



PLATE MX, FIGS 2-st, 36-30; and PLATE I. XXXIV. FIGS 1«:. IT. 



Avicula Boydii, Conrad. Jour. Acad. Nat. Sri.. Phila., vol. viii. p. 237, pi. 12, Eg. 4. 1842. 



Phiinea Boydi. Conrad, s. A. Miller in Cat. Am. Pal. Fuss.. p. 201. IsTT. 



Avicula quadiiila, Cohbad. Jour. Acad. Nat. Sci., Phila., vol. viii. p. 243. pi. 13, tit:. 5. Is42. 



Actinopteria Bvydi, Hall. Pal. N. Y.. vol. v. pt. 1. Plates and Explanations : PI. 19, 6ga. 2-24. 26-30 ; pi. 



23. Bgs. 5, B. Jan.. 1853. 

 Compare Avicula perobliqua, Cohbad. Jour. Acad. Nat. s.-i.. Phila., vol. viii. p. 23.">. pi. 12. ti;:. I. 1842. 

 " *' pleuroptera, Conbad. " " •• •• •• p 242, pi. 13. riir. 2. Is-pj. 



Shell of medium size, rhomboidal; body ovate, varying in proportions, 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 extended. 



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 defined sulcus. Wing- 

 large, triangular, not distinctly separated from the body of the shell : margin 

 concave; 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 

 15 



