204 Bulletin 39 376 



er. The surface of the valves appear nearly smooth but in well- 

 preserved specimens very faint concentric lines and fine radial 

 striae may be seen. The left valve is more coarsely sculptured. 

 The hinge is finely grooved with vertical lines. The measure- 

 ments given by Pilsbry for his Gatun specimen, length 2.8, 

 height 2.7, diameter of right valve .8 mm, are the usual dimen- 

 sions for the Costa Rican specimens. 



Gatun Stage: Gatun, C. Z. 



Coll. /, East Grape Point Creek. 

 Middle Creek. 



Pecten aotus, n. sp. Plate 18, figures 17, 18 



Shell small, thin, depressed or but slightly convex and 

 Ai'nusiuin-VCs.Q.; both valves except on magnification appearing 

 smooth; ears of the left valve large and subequal; the anterior 

 ear of the right valve about twice that of the posterior and car- 

 rying a deep, byssal notch; the surface of the left valve on mag- 

 nification is seen to be sculptured with fine, radial lines which 

 often occur in irregular streaks or blotches, imparting a peculiar 

 appearance to the shell; the right valve is perfectly smooth and 

 featureless, except for a band of rough scales on its anterior ear 

 above the byssal notch; interior of shell cavity shallow, a simple 

 ventral margin and the hinge-line vertically grooved with fine 

 lines. 



Length 4.5, height 4.5, diameter .75 mm. left valve. 



Differs from Pecten oligolepis Brown and Pilsbry in its larg- 

 er size, thinner and more AmHsium-\\\ie sheW, and strongly sculp- 

 tured left valve. This sculpture consists of fine lines or ca?np- 

 tonectes striations, which ma}^ be uniformly distributed but more 

 usually occur in irregular streaks or blotches, imparting a dis- 

 tinctive appearance to the surface. This sculpture is seen only 

 on a slight magnification, the shell otherwise appearing smooth 

 and featureless. It is quite common in the coralline limestones 

 of Port Limon. 



