300 Thirty -fifth Keport on the State Museum. 



ACTINOPTERIA DECUSSATA. 



Avicula decussata, Hall. Geolog. Surv. N. T.- Rep. Fourth Dist., p. 203. 1843. 

 Pteronites decussata, Hall. S. A. Miller. Cat. Amer. Pal. Foss., p. 202. 1877. 

 • Actinopteria decussata, Haul. Pal. N. Y, vol. v, pt. 1. Plates and Explana- 

 tions : PI. 17, figs. 24, 28 ; pi. 18, figs. 1-15. Jan., 1883, 



Shell large, sub-rhomboidal ; body sub-ovate, very oblique ; length 

 about one-fourth greater than the height; margins regularly curv- 

 ing; the anterior margin sometimes nearly vertical ; posterior mar- 

 gin more abruptly rounded. Valves convex ; the right valve less 

 convex and smaller than the left valve. Hinge-line straight, less 

 than the length of the valve. Beak acute, prominent, inclined for- 

 ward, situated close to the anterior end of the shell. Umbonal region 

 prominent, and in the left valve gibbous; subtending an acute 

 angle. Ear small, separated from the valve by a broad sulcus, 

 beyond which it is a mere fold in the shell. Wing large, triangular, 

 flat, extending nearly to the margin of the valve ; limited, in the 

 left valve, by a more or less distinct sulcus, and the abrupt bending 

 of the concentric striae ; margin concave ; extremity acute. This 

 character of the wing is somewhat less marked in the right valve. 



Test thick: the left valve marked with strong, prominent, 

 rounded radii, regularly alternating with finer ones on the posterior 

 half of the valve ; crossed at regular intervals by strong concentric 

 lamellae ; the interspaces marked by fine lines of growth. From 

 maceration or exfoliation these surface characters are usually only 

 partially preserved, or nearly obsolete. In certain conditions the 

 rays become nodose where crossed by the lamellae, and in many 

 examples the concentric striae interrupt the rays, leaving them 

 alternating above and below the lamellae. The surface characters of 

 the body are continued on the wing, while on the ear the concentric 

 striae are crowded and conspicuous, and the rays obsolete. In the 

 right valve the markings are much subdued, the rays often obsolete, 

 especially on the lower part of the valve, and the concentrie lamellae 

 are simple undulations of the surface. Ligamental area narrow, 

 linear; marked, apparently, by a single groove. The cast shows a 

 fold corresponding to the sulcus limiting the wing. Pallial line 

 extending from just anterior and below the umbo to the posterior 

 portion of the body and thence recurving, terminating in a small 

 oval muscular impression. No evidences of teeth are seen in the 

 casts or the interior of valves. 



A large specimen has a length of 76. mm., height 56 mm., hinge- 

 line 62 mm. A medium-sized example has a length of 52 mm., 

 height 43 mm., hinge-line 42 mm. 



