■1 - 



Shell small, well inflated, subequivalve, snbor'bieular, 

 slightly more prodxiced and tuieveiily roimded posteriorly; posterior 

 dorsal maro-in longer than the anterior; rig-ht valve sculptured with 

 twenty- tv70 or twenty-three angulr.r radial r ibs, the summit heing 

 slightly roimded and on adult shells the ribs roiinde-"'' at the ven- 

 tral margin, separated by angular interspaces, vath the apices of 

 the angles rounded; on the slopes of ^>' <^ ribs and in the bottoms 

 of the interspaces fine, closely spaced, sharply raised concentric 

 lines present, usually not rising above the summits of the ribs; 

 left valve sculptured with eighteen or nineteen similar, but broad- 

 er and more sharply angular, ribs and similar concentric lines; 

 submargins of both valves smooth e^ccept for very fine, inconspicu- 

 ous concentric striae; anricles subequal, "i th four or five narrov;, 

 unetiually sr)aced radial ribs, crossed by concentric imbrications; 

 cardinal crura conspicuoxis,. finely striated. 



Dimensions .- A left valve: lat., 14 ran.; alt., 14.1 nim. ; 

 diam., 3.2 ram.; length of hinge, 8.2 mm. A right valve: lat., 

 12..'! mm.; alt., 12.2 leu; diam., S mm.; length of hinge,, 6 mm. 



Remarks . - ^e two valves are of almost eraial co.ve"ity, but 

 the left displays a tendency to be slightly less convex and more 

 inequilateral. The radial sculpture on the tv/o valves is quite 

 different; on the left valve the ribs are fev7er, broarier and more 

 sharply angular, but still very slightly rounded, especially at 



