1:35 - 



erior and posterior lateral raar^-ins fee'bly incised on adidt shells; 

 anterior margin roimded; up-oer portion of posterior margin oblique- 

 ly trimoate"., lower portion produced; ventral margin sloping toward 

 the posterior end; sculDtiire of the two valves sirdlar, consisting 

 of about thirty, clear-cut radiating ribs, the one adjacent to the 

 antero-dorsal margin usually small and .arrow; the next eight to 

 tv/elve broad, being wider than the interspaces, and having a shal- 

 low, broad median sulcus; ribs in the centrr^l portion of the shell 

 narrow, usually less than half as wide as the inters'oaces, and 

 usually square; the posterior ribs, nine to eleven in number, much 

 wider than the intersr)aces, flat and with a narrow groove (rarely 

 two), almost or entirely obsolete on those ribs in the extreme post- 

 ero-dorsal region; concentric sciUpture of fine, closely set rais- 

 ei lines, producing on the sulcated anterior ribs a double row of 

 nodules and a sing'le row on the undivided medial ribs, the nodulat- 

 ion of the median ribs usually more conspicuous on the left valve; 

 posterior ribs not affected by the concentric sculpture; cardinal 

 area very narrow, v/ithone to three ligament grooves behind or ijnder 

 the ujnbones; teeth fine, closely set, short, vertical (except in 

 the posterior region), uninterrupted, but the me "'ian teeth very 

 short, inner margin of the valves deeply crenulatei and from the 

 crenulations faint lines, m.ost noticeable in the posterior region, 

 run up to war-' the umbo. 



