Maryland Geological Survey 643 



Actinoptcria hoydi Cleland, 1911, Bull, xxi, Geol Survey Wis., p. 109, pi. 

 xxli, figs. 7-8. 



Description. — " Shell of raedinm size, rhoniboidal; body ovate, varying 

 in proportions, the longitudinal axis forming 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; postbasal 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 ex- 

 tremity. 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 regul- 

 lar. sharp, elevated, concentric lamellre which (in good specimens) are 

 produced into subtubiilar, spiniform extensions upon the rays. Lines of 

 growth are seen between the lamellae. On the wing the rays are more 

 subdued while the concentric lamellte 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 ex- 

 pansions are conspicuous. In some casts they appear as undulating, 

 elevated lamella. Pallial line extending parallel to the margin of the 

 shell and terminating in a muscular impression on the posterior slope. 

 A small muscular impression is also seen just in front of the beak, and 

 obscure indications of one or two cardinal teeth. Ligameutal area nar- 

 roAV, striated, marked by two or three slender grooves, Avhich are slightly 

 divergent from the hinge-line." Hall, 1884. 



Length 20 mm.; height 15 mm. 



A single valve has been observed, which is probably of this species. 



