- Ih5 - 



Slope lov7, flat, relatively broad, and with one or two, usually 

 faint, lines; closely and evenly spaoe-^ coneentric lines present, 

 producing nodules on the ribs, e2:ceot on those on the posterior 

 slope and a few i!:^mediately anterior to it, the nodulations being 

 most conSDicuous on the proximal half of the ribs; inner margin 

 of the shell deeply crenulatei. 



Dimensions ,- Length, 59 mm.; alt., 45.5 mm.; semidiam. , 

 20 ram.; length of hinge- line, 40 mm. 



Remarks .- Individuals of this species are very abundant, in 

 fact, i.t is the most abundant Area , and all stages of growth are 

 represented. Yoimg forms have a shape that is noticeably different 

 from that of the adults; the inper part of the posterior lateral 

 margin is not obliquely tru: cated or the truncation is not oro- 

 noujice'"! and the lower part of the same margin is not as much nro- 

 duced, so that the angle between the postero-dorsal and postero-lat- 

 eral margins is less obt\;se, and the entire posterior lateral mar- 

 gin Dtraighter. Other differences between young and adult are 

 apparent when details of sculpture are consideree. On adolescent 

 shells the svilcus aT)T)ears on only a few Df the anterior ribs and 

 the lines on the posterior ribs disappear; #iile on young shells 

 the ribs in even the extreme anterior portion of the shell are not 

 sulcated, in other words, there is a progressive diminution in 

 the number of anterior ribs that display siil cation. Some immature 



