126 Studies in Geology, No. 3 



sharp concentrically channelled threads which extend across 

 the part next the submargin in reversed curves, making the 

 complete threading S-shaped ; byssal notch prominent ; com- 

 parative length of ear not known. Posterior ear subrec- 

 tangular, with sculpture of very fine, closely spaced concen- 

 tric threads and very faint radial ribs. Sculpture on the 

 disc of very fine growth-lines which cross the ribs and inter- 

 spaces in smooth circles. Inner shell fluted. 



The left valve similar to the right ; anterior ear orna- 

 mented by about six somewhat widely spaced radiating ribs 

 which are crossed by fine lines of growth ; outline unknown. 

 Posterior ear unknown excepting a fragment near the sub- 

 margin, which shows several radiating ribs crossed by very 

 fine lines of growth. Sculpture on disc of fine growth-lines 

 which curve upward on the ribs and downward on the inter- 

 spaces, giving the threading a sinuous aspect. Both valves 

 have apical angles of 90° or more. The margins form 

 approximate semicircles. Dimensions : Right valve, length, 

 37.5 ; height, 36.25 mm. Left valve, length, 32 ; height, 

 30.75 mm. 



This species is very close to a group described by Cooke 

 in 1918 from the Oligocene of Cuba and Anguilla. Though 

 smaller, P. gardfierae,^-'^ from Anguilla, is perhaps closest 

 of the three similar forms ; it dififers in being more inflated, 

 smaller, with higher ribs, and in having five ribs instead of 

 four on the anterior ear of the right valve. P. vaughani 

 var. ftabcUum Cooke ^"" (not of Arnold 1906), also from the 

 Oligocene of Cuba, has similar ribs, but dififers in being 

 smaller and in that the ribs do not extend to the beaks. This 

 character of the ribs is the chief dififerential feature between 

 the variety Habclhim and Pect en vaughani Cooke ^^^ (not 

 Arnold, 1906), but that species dififers from woodringi in 



*^ Carnegie Inst. Wash., pub. 291, p. 134, pi. 7, figs. 5, 6, 1919. 

 ""Idem., p. 134, pi. 8, figs. 8, 9, 10. 

 "Mdem., p. 133, pi. 8, figs. 2, 3, 4. 



