220 Thirty-fifth Report ok the State Museum. 



A-VICULOPECTEN ClEON. 



Amculopecten CUon> Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations : 

 PL 1, fig. 1. Jan., 1883. Compare with the young of A.princeps. 



Shell small, ovate ; transverse axis slightly oblique ; length some- 

 what greater than the height ; margins regularly rounded. Left 

 valve convex, the convexity equal to about one-sixth the length of 

 the hinge-line. The point of greatest convexity is just anterior to 

 the center of the shell and toward the beak. Right valve not ob- 

 served. Hinge-line straight, length five-sixths the height of the 

 entire shell. Beak anterior to the middle of the hinge, not promi- 

 nent. Ears defined by the absence of radiating lines, but not by any 

 marked change in the convexity of the shell ; lateral margins con- 

 cave. The anterior ear is limited by a shallow groove. The posterior 

 ear is about twice the size of the anterior. 



Test thin, ornamented by a reticulation of fine radiating lines, 

 and finer concentric striae of growth. The .stronger rays have one 

 or two finer lines between. The lines of growth extend over the 

 ears, but less distinctly than over the body of the shell ; and other- 

 wise the ears are nearly smooth. The length of the specimen is 21 

 mm., height 24 mm., hinge-line 20 mm. 



This form resembles the young of A. princeps, but it is less oblique 

 and the ears less strongly defined than in the yoitng of that species as 

 represented in authentic specimens. The form A. exacutus is very 

 similar to this one ; but that species has fewer and sharper radiating 

 striae, and the geological horizons of the two are widely separated. 



Formation and locality. Upper Helderberg limestone, Columbus, 

 Ohio. 



AVICULOPECTEN SCABEIDUS. 



Aviculopecten scabridus, Hall. Pal. N.T., vol. v, pt. 1, Plates and Explana- 

 tions : PI. 3, figs. 3-12. Jan., 1883. 



Shell large, sub-circular, very slightly oblique to the hinge-line ; 

 length somewhat greater than the height : pallial margin circular, 

 becoming very full posteriorly. Valves depressed, moderately con- 

 vex. Hinge-line straight; length equal to two- thirds the length of 

 the shell, marked by a thickening and inflection of the cardinal mar- 

 gins of the ears. Beaks obtuse, rising slightly above and arching 

 over the hinge-line, situated just anterior to the middle; umbonal 

 angle 110°. Ears narrow, triangular, indented by a sinus which 

 has a depth equal to the width of the ears. The anterior ear of the 



