FOSSILS OF THE SILURIAN AND DEVONIAN ROCKS. 229 



tween the body and the posterior wing is less strongly defined. These are the 

 differences marked out by Prof. Hall. It appears to me that in all these points 

 shells may differ and still belong to the same species, and especially in this 

 case, where the most important distinction, the last mentioned one, disappears, 

 inasmuch as Prof. Hall did not notice in his specimen the dividing sulcus be- 

 tween body and posterior wing, which my shell so plainly shows. 



Formation and Locality. Found in the Corniferous limestone at and around the Falls of the Ohio, 

 in Kentucky and Indiana. 



Genus Actinopteria. 



This genus is established by Prof. Hall in his N. Y. Rep., Vol. 5, part 1, 

 Lamellibranchiata. His description is : 



Characterized from Pterinea in the absence of a broad striated ligamental 

 area and strong cardinal teeth. Eight valve sub-convex. Surface with tine 

 rays. 



Actinopteria boydi. CONRAD. 



Plate III., figure 2. 



Avicula boydi , Conrad. Jour. Acad. Nat. Sci. Phil. 1842. 

 Avicula quadrula, Conrad. Jour. Acad. Nat. Sci. Phil. 1842. 

 Actinopteria boydi, Hall. Pal. N. Y., Vol. 5, pt. 1, p. 1131883. 



Shell of medium size, rhomboidal ; body ovate, varying in the proportion 

 of its dimensions. Its longitudinal axis forms with the hinge-line an angle 

 of from forty-five to sixty degrees. Length and height sometimes nearly 

 equal, but in some specimens the length is one-fourth greater than height. 

 The margins in the basal half of the shell regularly rounded; in the upper 

 half of the valve anterior margin of the body is almost straight or very little 

 curved, while the posterior border of the body is entirely straight or even 

 slightly concave. The post-basal part of the shell is extended. The left valve, 

 which is only preserved in the specimen before me, is moderately convex 

 throughout the whole body. The umbo is somewhat more elevated; its ante- 

 rior slope is gentle, while the posterior descent is abrupt, meeting surface of 

 the posterior wing almost at right-angles, thus forming a sulcus by which 

 the rear wing is defined. 



The hinge-line is straight from the anterior side of beak to the posterior ex- 

 tremity. Beak anterior, acute, prominent, inclined forward, and rising above 

 the cardinal 'line ; the umbonal margins form an acute angle of less than 

 sixty degrees. Front ear short, oblique, limited by a deep, but not sharply 



