- 193 



Shell thin, width and. length almost equal, hinfe line aporoxi- 

 nately equp.l to half the width of the shell; dorsal marp-invS Ion?, 

 slightly conoave, diverging" at an angle of about 105°; ventral half 

 of the outline forming an evenly ser.icirculQr ; on the very convex 

 right valve the radial lines hetv/een the obsolete "ribs" but feebly 

 iraoressed, the surface more or less polished, with very faint, ir- 

 regularly space' incremental lines, usually slightly more conspi- 

 cuous and more regularly dispose! on the submarrins and on certain 

 irregiiler, somev7?:at worn areas on the disk; the concave left valve 

 ■•:ith the umbonal portion almost smooth, the intfrsoaces between 

 the low ribs usually slightly raised medially; the conoentric fine 

 lines more conspicuous than on the right, though still faint, and 

 ■:iore regularly spaced; auricles on both valves subequal an-^ smooth, 

 except for fine incremental lines, the lines decidedly -inuous on 

 the right anterior a-^iricle; on the interior of the left vslve the 

 ■laired lirae visible for only a short distanse from, the mar~in, 

 "-:e interval between the two members of a oair more deeply channel- 

 led than the interval between two pairs, the onoosite of the condi- 

 "^ion on the right valve, with lirae extending further up toward 

 the vimbo. 



' Dimensions .- A right valve.- lat.* 44 mm.; alt., 45 mm.; 

 'Ian;., 15 mm.; length of hinge, 21 mm. A left valve.- lat., 

 47 ^::m. ; alt., 46. S m .; :''iam. , 6 mm.; length of hinn-e, 22.5 mni. 



