- IIS - 



AR3A (STEITOAI^OA) SUBOAITDEIDA n. sp. 



Descrintion.- Shell small, -fcransverselv r-nibellim'Tloal. 

 hioiher and more producefl posteriorly, slightly? inflated; posterior 

 dorsal margin direct; rounded posteriorly, the curve of the upper 

 portion being less convex; subtruncated anteriorly; ventral margin 

 sloping gently downward toward the posterior end; umbones veyy low 

 and flat, strongly prosogyrate, situated at about the anterior 

 fifth of the length; posterior ridge low and very broad in the 

 umbonal region, becoming obsolete toward the marjrin; external sur- 

 face sculptured with numerous fine radial ribs, narrowest in the 

 region below the ujnbo and widest posteriorly; the ribs separated 

 by fine impressed lines except on the posterior slope where be- 

 tween some of the ribs ^re interspaces as v.'ide as the ribs them- 

 selves; concentric sculpture of regular, closely spaced concentric 

 impressed lines, overriding the radials, thus producing a conspicu- 

 ous reticulated sculpture and causing the ribs on the posterior 

 slope to be nodixlated; cardinal area narrow, irapressed, almost en- 

 tirely opisthodetic^ the very short portion in front of the umbo 

 being widest; hinge as in A. dent era n. sp., but with five teeth 

 in the anterior series and eighteen in the posterior; inner mar- 

 s-in of the valve smooth. 



Dimensions .- length, 13 mm.; umbonal alt. 7.6 mm.; max. 

 alt., 9.2 mm.; semidiam.. , 2.8 mm.; length of hinge, 7 ram. 



