172 Bulletin 39 344 



and a widely truncated anterior end; valves gently convex; pro- 

 toconch on the tip of the umbo small, smooth and white; surface 

 polished; the sculpture of the disk of the shell is developed to a 

 variable extent, but the umbos are always smooth; typically the 

 lower half of the shell is covered with close, regular, concentric 

 wrinkles which are slightly decussated by fine, even, and slight- 

 ly raised radial threads; in other cases, the wrinkles may be pres- 

 ent only on the extreme anterior or posterior ventral extremities 

 and the radial lines showing only as a part of the shell substance 

 and not as raised threads; the escutcheon is elliptical and thickly 

 covered with raised wrinkles which lie at right angles to the dor- 

 sal margin of the shell; they are similiar on the lunule; interior 

 of the shell tinted with brown; ventral margin crenulated; chon- 

 drophore small, oblique with the anterior set of teeth numbering 

 about 7, the posterior about 14. 



Length 3.50, height 3.00, diameter 1.60 mm. 



This is a small species of subtrigonal outlines, suggestive of 

 the Pacific N. exigua Sowerby in general form and size. On the 

 few specimens of N. exijpia, which I have from the Bay of Pan- 

 ama the sculpture is finely decussated over the whole shell with 

 fine radial and concentric threads. The most striking features of 

 cahuitensis are its strongly sculpture lunule and escutcheon. 



Gatun Stage: Zo7ie G, Saury Creek. 



Nucula tenuisculpta Gabb Plate 18, figure 25 



Nulua tenuisculpta Gabb, 1873, Trans. Ainer. Phil. Soc, vol. 15, p. 



255- 

 Nucula teuuisculpta Maury, 1917, Bull. Amer. Pal., vol. 5, p. 325, pi. 



26, figure 8. 



A species of the size and form of cahuitensis but the valves 

 are entirely smooth and polished. The lines of growth are fine 

 and irregular. The radial lines are faint and sub- microscopic. 

 Inner margin crenulated. 



Height 3.25, length 4.00, semidiameter .75 mm. 



Gatun Stage: Coll /, Red Cliff Creek. 



