CLFNTON, LOWER HELDERBERG, CHEMUNG, AND WAVEHLY GROUPS. 445 



Surface marked by thirteen plications, of which five occupy the mesial sinus, the 

 thi-ee central ones being larger than the outer ones, but all smaller than those on 

 tlie other portions of the shell ; plications subangular or angular. There are also 

 very fine radiating striie on the plications, three in the space of 1 mm. The plica- 

 tions are ci'ossed by fine concentric sti'iae, which are most prominent on the fi-ont of 

 the valve, becoming obsolete above, and also by stronger imln'icating lines or vari- 

 ces of growth. 



The specimens observed have a height of about 35 mm.; width just below the 

 middle equal to the height. 



This species may be distinguished from any other of this formation by its large 

 size, deep sinus and radiating striations. This species closely resembles lihyncho- 

 nella missoitriensis of the Kinderhook group, but the shell is larger, the apex more 

 pointed, giving an angular appearance to the upper portion of the shell, and the 

 greatest width is below the middle. 



Formation and locality. Waverly group, four miles south-west of "Warren, 

 AVairen county, Pennsylvania. 



AVICULOPECTEN .-EQUALATA Simpson, n. sp., Kig. 11. 



Shell of medium size, equilateral, not oblique, nearly circular ; length a little 

 more than the width ; basal and lateral margins below the ears regularly and broadly 

 rounded. 



Left valve flattened; greatest convexity a little more than one-fourth the length 

 of the shell from the beak ; very gradually sloping to the \ ~ "~~~' "^ i 



lateral and basal margins. '"'^ ^^ 



Kight valve unknown, /^r; 



Hinge line straight; length a little more than two- , 

 thirds the width of the shell, central, not extending as 

 far as either lateral margin. 



Beak obtuse, I'ounded, straight ; umbo ]>rominent, 

 well defined by its slightly gibbous form and its rapidly 

 sloi)ing aides. \, . „c ^r >«„ 



' o "v.' A ns. Tr. 1809 



Ears subequal, narrow, triangular, and each separated from the body by a nar- 

 row sinus; lateral margins convex. 



Surface ornamented by narrow but distinct rays of essentially unil'onu size ; at 

 the base there are six or seven in the space of 5 mm. ; a little above the middle of 

 the shell there ai'c twelve in the same space ; interspaces wider than the rays. The 



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