- IGl - 



Long. 35, alt. 53, diam. 22 mm." (Dall, 1898). 

 [gype local i't-y .- Bow den, t7araaica. 



Shell decidedly convex; umljones lov;, decidedly inflated; adults 

 with the distal two-thirds of the posterior slope gently keeled, 

 the point of impingraent of the keel on the posterior lateral rjarrin 

 heiiog the dorsal termination of a trimoation extending ohliqnely 

 dOL-mward to the postero -ventral margin, then Joining the even curve 

 of the anterior half of the ventral mar-in; anteri^i-ly the straight 

 dorsal margin passing rapidly into the broad curve of the azitero- 

 lateral margin; the slope between the postero-dorsal and postero- 

 lateral raaro-ins less abrupt; external sculpture of many very lov;, 

 inconspicuous radiating ribs, narrowing into more prominent elevat- 

 ed lines on the umbones and absent in the extreme antero- and post- 

 ero- dorsal regions; fine, equally spaced radinting striae and enn- 

 ally fi;.e, but less prominent, concentric striae superimposed on 

 the ribs and narrower interspaces, the two producing a minutely can- 

 cel! ated sculpture visible only on unworn valves; cardiral area 

 very narrow; hinge-plate gently and imiformly arched; teeth relat- 

 ively small, usually 12 before and IS behind, the umbo; buttresses 

 of the muscle-scars not prominent, 



Dimensions .- Length, 55 mm.; alt., 7,^- rm. ; semidiam. , 11.5 mmi. 



Remarks . - iais species is most abundant of the miembers of the 

 genus. Individuals that have assumed adult features are character- 



