- IC' - 



anteriorlv and ventrally; umoones very low; external sculpture 

 consisting of .abo^it twentj-'-six lov/^ radiating rilos, sharply anp-ular 

 on imv/orn shells except in the extreme antero- and postero-dorsal 

 rep-ions, the flanlcs of the rihs sloping gently from the angular 

 sxinniit, forming angular interspaces; concentric sculpture of fine, 

 equcally spaced striae, usiially not overriding the summits of the 

 radials , hut Gonspicuo^^s on their flanks; cardinal area narrow, 

 very short, not affecting the angular outline of the dorsad marpin, 

 Gonsnicuously marlred hy ligament grooves diverging from a median 

 vertical line; hinge plate decidedly arcuate; teeth small, numerous, 

 the series interrupted by the subsidence of the cardinal area, 

 with twelve or thirteen teeth on either side; inner margin of the 

 valve coarsely and weakly erenulated. 



Dimensions .- Length, 24 mm.; alt. 25.8 mm.; semidiam., 6.5 w:.. 



Remarlts .- The beautifully sculptured shells of this widely 

 distributed species are almost as nimierous as jamaicensis . The 

 low, sharply angular ribs arjd subangular posterior yroduction are 

 the salient characters. Among immature forms the irregular outline 

 of the oosterior margin is poorly defined or entirely lost. Young 

 shells apparently be!J.onging to this species, present puz-ling 

 features. Some have the typical, somewhat compressed form and char- 

 acteristic angular scul"Dtire; others are more inflated, with high- 

 er umbones, angiilar or subangular ribs, separated by definite in- 

 terspaces; another group, including most of the sm.alTest shells. 



