LAMELLIBRANCHIATA. 37 



AviCULOPECTEN ORNATUS. 



PLATK II, FKiS 7, S; AND PLATE III, FI(i. U. 



AvU-idiipecten oruatu.i. Hall. Pal. N. Y.. vol. v, pt. 1. Plates and Explanations: PI. 2, tigs. 7, 8; pi. 3, 



tiff. 14. Jan., 1883. 



Shell small, longitudinally oblique-ovate, sub-rhomboidal ; length and width in 

 the propoi'tion of three to four ; margins regularly rounded. 



Valves depressed-convex; left valve somewhat more convex than the 

 right, and a little larger. 



Hinge-line straight, slightly less than the length of the shell, central with 

 regard to the body of the shell. 



Beak obtuse-angular, directed forward, situated a little anterior to the 

 middle. 



Ears sub-equal. Posterior one the larger, triangular, flat or concave, 

 defined by the abrupt post cardinal slope of the unijjo ; extremity acute ; 

 margin concave. Anterior ear defined by a deep sulcus ; byssal sinus deep, 

 more strongly marked in the right valve. 



Test of the left valve ornamented by about 25 strong, sharp, elevated rays, 

 and wide interspaces which are marked Avith a smaller ray ; crossed by 

 undulating, concentric stria3 which are curved backwards between the 

 stronger rays, producing arched fimbria?. The right valve shows the 

 radiating striae originating near the umbo, which is smooth, jiiid increasing 

 by a double bifurcation, giving them a fasciculate appearance ; they are 

 crossed ])v tuidulating, concentric striaj. The ears are marked by strong, 

 radiating lines and fine, concentric stria?. 



Internal characters not observed. 



The length of the largest specimen is 18 mm., height nearly 15 mm., 

 and hinge-line about 17 mm. 



This species, in form and general appearance, is very similar to A. bellus ; it 

 differs conspicuously in the ornamentation, with proportionally shorter posterior 



