- 180 - 



Dimensions.- A longitudinally sulDOvate right valve: lat. 

 86.5 nim. ; alt., 40.5 nm. A subfaloate right valve: dorsal lat., 

 17 mm.; ventral lat. 27.5 mni. ; alt., 31.5 mm. 



Remarks .- The proverbial difxioiilties iiivolved in the diff- 

 erentiation of the members of this genus are augmented in the case 

 of the Bo'.7den osisters by the almost complete absence of lar^re in- 

 dividuals. The s-oeoimens referred to £. me go don are b:y far the 

 most numerous. Typically this species is subfalcate, but most 

 of the indivirluals are only slightly curved, '.iiile many of them 

 have an o -.tline vvhich varies from longitudinally? elongate to lo2ig- 

 ituciinally subovate ^ith the umbo placed at or near the posterior 

 end. The most characteristic features are the small number of 

 broad, subrounded, short plications on the left vr.lve, the similar 

 sculpti^re or irregular warping of the right valve, and the absence 

 of laminatei concentric sciilpture. The somev/hat falcate 0. 'oauci - 

 Dlicata Uall (l), described from the Oak Grove s?.nd, has a larger 



(l). Trans. 'Vagner 7ree Inst. Sel . , Philadelphia, vol. ?, pt. 4, 



p. 578, 1398. 'As _0. sellaeiormis ^onrad vsr. pauoiplieata . ) . 



number of radials (seven to fifteen) and regular concentric lami- 

 nae. !^on- falcate forms of megodon som.ev/hat resemble the v/idespread 



