343 Costa Rica. Miocene — Olsson 171 



ORDER PRIONODESMx\CEA 



Genus NOCULA Lamarck 



Nucula ori3ice]Ja, n. sp. Plate 28, figures 19, 20 



Shell of medium size, broadly elliptical or subovate in out- 

 lines; obliquely truncated at its anterior end; gently convex; the 

 surface is covered with close, subregular, concentric wrinkles and 

 fine radial threads so that the sculpture is decussated; on the 

 posterior dorsal slope, they are larger and pustular-like and their 

 course is obliquely downward across the growth-lines; they are 

 weak on the poorly defined lunule; the radical threads are main- 

 ly between the concentric wrinkles but ventrally they cut into 

 the wrinkles as well; interior of the shell, silvery white or faint- 

 ly tinged with purple; the ventral margin is finely crenulated; 

 chondrophore very small, oblique; anterior set containtng about 8 

 teeth, the posterior with about 17. 



Length 9.50, height 7.50, semi-diameter 2.25 mm. 

 11.00 8.50 2.50 mm. 



From the recent N. crenulata A. Adams, this shell differs 

 by its larger size and more elliptical and depressed shell. Gabb 

 has described from the Pliocene of the Limon Peninsula Nu- 

 cula limonensis and moeneiisis, but both of these species are per- 

 fectl}^ smooth and sculptureless. The Nucula orbicella is a com- 

 mon species in certain marly beds intercalated in the coralline 

 limestones of Port Liraon. 



Gatun Stage: Port Limon. 



Nucula cahiaJteJisJS, n. sp. Plate 18, figures 21-24 



Shell small, solid, obliquely subtrigonal in form; ventral 

 margin widely rounded, a nearly straight posterior dorsal margin 



