373 



Costa Rica Miocene — Oi^sson 



20 1 



large examples of levicostatiis. The sculpture of the posterior 

 ear is uniform. In the Chasapeake Miocene fauna the Peden 

 eboreus Conrad, variety coniparilis Tuomey and Holmes is dis- 

 tantly related, but is larger, less convex and generally with 

 more numerous ribs. 



Gatun Stage: Gatim, C. Z. Quitana Creek. 



Water Cay. 



Ho7ie Creek. 



Banana River. 



Estrella River. 



Pecten ccstaricensis, n. sp. Plate 18, figures 8-1 1 



Shell small, inequivalve, oblique; the right valve is moder- 

 ately convex, the left but slightly; right valve with 17 fiat-top- 

 ped ribs, square in section and with deep U-shaped interspaces 

 of a width equal to the ribs; these interspaces are sculptured 

 with fine, concentric, raised lines which are quite far apart on 

 the umbos, but become crowded ventrally or disappear entirely; 

 the left valve has 17 or 18 ribs which are lower and more round- 

 ed in section; the fine concentric lines on the left valve are con- 

 fined to the umbos, the interspaces over the greater part of the 

 shell being smooth; the anterior and posterior dorsal slopes are 

 flattened and smooth, but sculptured below in each valve by 

 strong radial threads on the lower side of the ears; ears subequal; 

 the right valve with a deep, byssal notch in its anterior ear; the 

 left valve has the lower half of each ear with 3 strong radial 

 threads, while on the upper half, the radial threads are largely 

 obsolete; valves internally grooved in harmony with the exter- 

 nal ribbing to about the center of the shell. 



Height 26.50, breadth 28, thickness 7.50 mm (right valve) 

 24 24 4.00 mm (left valve) 



27 28 5.50 mm (left valve) 



Differs from the Pecte7t levicostatiis Toula, by its unequal 

 valves, less convexity and fewer ribs. It is somewhat like the 

 Pecten inceqiialis Sowerby, from which it is distinguished by its 



