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ARGA (OALIOAHGA) liKDOIvIENA n. sp. 



Description .- Shell small, elongated, subtrapez^idal, deci- 

 derlly inflated; dorsal and ventral rnar'-ins subparallel, the ventral 

 maro:in slightly arcuate, with a broad, feeble insinuation toward 

 the posterior end; rounded anteriorly?; oosteriorly truncated, pro- 

 duced below; umbones tumid, broad, situated at about the anterior 

 third of the length; posterior ride:e rou.ided, but prominent; er- 

 ten.al surface sculptm-ed with nxicierous radial ribs; the extreme 

 anterior ribs separated by interspaces of virtuall: the sanie width, 

 the succeedir]g ribs narrower, with the interspaces wider than the 

 ribs and often bearing a fine secondary radial; toward the post- 

 erior ride-e the ribs become wider and are separated by narrow im- 

 pressed lines; on the posterior slope ribs few and broad, inter- 

 spaces of about the same width; the radials beariiig obscure elev- 

 ations along concentric lines, exoeot in the anterior portion of 

 the shell where the concentric sculpture forms definite nodules; 

 cardinal area morJerately wide, the posterior portion v/ith oblique- 

 ly directed ligament grooves; hinge bearing about twenty-five 

 teeth, the posterior ones notably oblique, the anterior less ob- 

 lioue, the series uninterrupted i-edially; posterior and anterior 

 inner mare:ins obscurely crenulated. 



Dimensions .- Max. length, 10 mm.; alt., 5.5 nmi. ; semidiam.., 

 2.5 mm.; length of hinge, 6.5 mm. 



