Shell larpe, he'-vj;, moderatelj- inflated, elomate, inequilat- 

 eral; left valve slip'htly smaller; rouj^ded anteriorly, posterior- 

 ly the r;.ar?-in curving gently to the shano oostera-ventrsa anirle; 

 base p-entlj? arcuate, gently enargins-teri. oosteriorly; vmbo low, 

 hroad, place 1 anterior to the rne'iaj.i vertical; limular depress- 

 ions deep, subspherical, larger on the left valve; posterior car- 

 ina sharp, more pronomiced ventrally, cituated near the posterior 

 rnargin, the -oosterior dope narrow and flattened; external sur- 

 face sculptured with coarse concentric rugae, suppressed before 

 the posterior carina is reached, with fine concentric striae in- 

 tercalate" betv-'een the ru^-ae and c "■'"*■':;-■ nous across the posterior 

 slope and the area in front of itj racial sculptirre of fine, ob- 

 scure, distant radial striee between the rugae, the rs-i.ials ab- 

 sei:t on the posterior slope; the cardinal tooth in the right valve 

 strong. 



Piroensions .- Lenjrth, 17.5 rrn. ; sit., 12 nun.; seraidiam. , 

 4.8 ram. 



?.emar>s .- This s-necies, v,hich v/ell-represented, is reriark:- 

 able for the depth of the lunular deiression, on the bs.sis of 

 v/hich '""-abb erecte'^'- as a set)'rate genus ^.othrocorbnl^, . ^-Iie faint 

 radial scul-iture is often obsolete or even absent. The larc'est v 

 valve has the following dimensions: length, 20 mm.; alt., 15 mm.; 

 cl i am . , 7 rm. 



Dall has described several closely related S)»cies,- the 

 Ohipola 0. synarmostes (l), the Oak Grove 0. radiatul a (2) and 



