242 Thirty-fifth Report on the State Museum. 



AviCULOPECTEtf BELLUS. 



Aviculabella, Conrad. Ann. Geolog. Rep., N. Y., p. 54. 1841. 

 Aviculopecten bettus (Conrad), Hall. Pal., N. Y., vol. v, pt. 1. Plates and 

 Explanations; PI. 2, figs. 5, 6, 9. Jan., 1883. 



Shell small, longitudinally sub-elliptical ; height equal to two-thirds 

 or three-fourths of the length ;. margins regularly rounded. Valves 

 moderately convex ; the right valve somewhat shallower than the 

 left; very similar in form and ornamentation. Hinge-line straight, 

 the length a little greater than the longitudinal diameter of the 

 shell, nearly central, extending equally between the posterior and 

 anterior margins. Beaks obtuse-angled, directed forward, placed 

 anterior to the middle of the valve ; sides of the umbo carinate, and 

 descending abruptly to the ears. Posterior ear flat or concave, nar- 

 row-triangular, well-defined by the carinate sides of the umbo, and by 

 the absence of the strong surface radii ; margin concave ; extremity 

 acute, mucronate. Anterior ear about two-thirds as long as the pos- 

 terior one, triangular, convex ; defined by a deep sulcus, and the angu- 

 lar cardinal slope of the umbo ; margins of the ears in the left valve 

 straight or concave ; in the right valve, convex; extremities acute- 

 angled. In the left valve the byssal sinus is broad and rounded ; in 

 the right valve it is a deep, angular notch. 



Test of the left valve marked by from 40 to 50 regular, continuous, 

 thread-like rays, alternating in fours, with three degrees of promi- 

 nence, crossed and crenulated by fine, regular, concentric striae. 

 The surface of the right valve is nearly the reverse of this, having 

 broad, flattened rays, arranged in pairs, regularly bifurcating, with 

 narrow concave interspaces which correspond to the rays of the 

 opposite valve. The posterior ears show several delicate rays extend- 

 ing from the apex of the beak over their upper portion. 



A specimen preserving both valves has a height of 14 mm., length 

 19 mm., and hinge-line 20 mm. A right valve has a height of 14 

 mm., length 22 mm. A left valve has a height of 12 mm., length 

 16 mm., hinge-line 17 mm. 



This species in appearance resembles A. ornahcs, and differs in its 

 longitudinally narrower outline, more obtuse beak, the numerous rays, 

 and the absence of strong, lamellose, concentric fimbrise. The two 

 forms are of a group resembling several species in a parallel group of 

 the genus Actinopteria, which are remarkable as possessing an 

 aviculoid character (when compared with some recent species of 

 Avicula) ; in the strong rays, the deep anterior sulcus and byssal sinus 

 separating the anterior ears from the body of the shell. 



