1911.] NATURAL SCIENCES OF PHILADELPHIA. 361 



circular waves, and excessively fine, close radial strise. Internal 

 process narrow and slightly asy mmetrical at its insertion. 



Fig. 2. — Cheilea princetonia, dorsal and lateral views of internal cast. 



Width S.8, alt. 3.5 mm. 



The type is an internal cast and external mould of the same individ- 

 ual, in a hard matrix. The radial striation is finer than in the recent 

 Antillean C. equestris of the same size. C. vario (Brod.) of the Pacific 

 coast is apparently more nearly related to the Isthmian form. 



The type is No. 5,516 of the Princeton University museum, col- 

 lected by Mr. Ward H. Farrington, 1908. 



NUCTJLlDiE. 

 Nucula (Acila) isthmica n. sp. PI. XXVII, figs. 11, 12. 



The shell is trigonal with the upper margin anterior to the beaks, 

 and the basal margin strongly convex, the posterior margin shortest 

 and straight; posterior angle rather acute, the anterior rounded off, 

 beaks moderately prominent, opisthocoelous, at the posterior third of 

 the length; no lunule. Externally an angle, strong but not acute, 

 runs from the beak to the posterior angle, and a depression or con- 

 cavity of the outer face accompanies it. Elsewhere the outer face 

 is strongly convex. The flat, truncate, posterior area has a sculpture 

 of arcuate ribs, divaricating at the angle from those of the outer face, 

 and terminating at right angles with the posterior end. The dorsal 

 area anterior to the beaks has a corrugation composed of short ribs 

 which run almost transversely to the long axis of the shell. The rest 

 of the surface has close radial sculpture of riblets which diverge V-like 

 from a median line. These riblets are a little irregular, in places 

 slightly tuberculiferous ; they are closer and a trifle narrower near the 

 posterior concavity, becoming perceptibly more spaced anteriorly, 

 where the intervals are a little wider, crossed by irregular, fine, raised 

 24 



