Lamellibranchiata. 273 



This species differs from P. Icetus, by its distinct and widely sepa- 

 rated rays, and more oblique outline. 



Formation and locality. In the Chemung group at Tioga, Tioga 

 county, Pa. 



Pterinopecten dispandus. 

 Pterinopecten dispandus, Hall. Pal. N. Y., vol. v, pt. 1. Unpublished. 



Shell large, somewhat rhomboid-ovate, oblique, varying with the 

 age of the shell ; length a little greater than the height ; margins 

 regularly curved, extending behind. Left valve convex. Right 

 valve depressed-convex, somewhat smaller than the left, and pro- 

 portionally longer. Hinge-line straight, less than the greatest 

 length of the shell, nearly central. Beaks obtuse, little elevated, 

 anterior to the middle, directed forward. Umbonal region scarcely 

 defined, moderately prominent, subtending a right angle. Ears 

 triangular, undefined; margins concave. Posterior ear larger than 

 the anterior. Anterior ear limited by an undefined sulcus and mod- 

 erate sinus ; extremity rounded. In the right valve the anterior 

 ear is narrow, acute, with a deep, angular byssal sinus. 



Surface ornamented by prominent, rounded radii, which alternate 

 irregularly with finer intercalated radii, and increase in size and 

 number toward the margin ; crossed by concentric striae, with more 

 distant varices of growth which crenulate with radii. The rays are 

 stronger over the anterior slopes, and are continued on the ears, 

 somewhat finer on the posterior and stronger on the anterior. In 

 the right valve the- strong rays are regularly duplicating, and are 

 finer and equal on the posterior ear, with a few stronger ones on the 

 anterior ear. Ligamental area narrow. 



Two left valves measure respectively 45 and 37 mm. in length, 

 42 and 44 mm. in height, hinge-line 40 and 30 mm. A right valve 

 is 32 mm. in length, 27 mm. in height, hinge-line 28 mm. Other 

 specimens of the left valve vary considerably, apparently according 

 to age. 



This species differs from P. Vertumnus in its greater proportional 

 height, shorter hinge-line, abruptly rounded radii, and much sharper 

 small intermediate striae. The radii upon the right valve are also 

 stronger and distinctly bifurcating: the concentric striae are stronger, 

 closer, and crenulate the rays more distinctly. P. suborMcularis is less 

 oblique and the form more rounded, while the rays are more numerous 

 and closely arranged. 



[Sen. Doc. No. 38.] 35 



