- F51 - 



Shell very small, siibovr.- e-trigonal, decidedly inflated in 

 the Tim"bonal region, sulDequilateral; the av.we of the lov/er T!or- 

 tion of the posterior •'-r~in slin-htlj' more conve:: than the corres- 

 ponding part of the anteri-^r margin; base gently arcuate; iimbones 

 relatively prominent; exterior sia-face of the disk scTilr)tnred '.vith 

 foirteen to sixteen strong radial ribs, on immature sVjells rounded 

 throughout, but on adults beoom.ing angular in the dis'al third 

 of their length and at the same ti~;e bearing minute nodulations 

 on their sides; ribs seoarated by interspases of virtually the 

 same v/idth; thro^ighoiit almost the entire length of each inter- 

 S"oace a smsll, median, rouridec!, minutely nodulated ridge nresent; 

 submar~ins sculpture" with ten to fifteen concentric rn.gae, the 

 posterior subm.argin longer and usually having the concentric rug- 

 ae more prominent and more nLTjnerous; ligament pit small, shallow, 

 subumbonal v.lth six to eight teeth on either side; inner margin 

 of the portion of the shell v/ith radial sculpture deeply crenu- 

 late "" ; in"^erior if the shell ^dth inconspicuous rrdial nodula- 

 tions corresponding to the external radial sculpture. 



-^inensi "'ns .- Lt., 7j nm. ; alt., 3.2 mm.; semJ.diam., .9 mm, 

 •'•^em.ar'rs .- ''"his interesting srnecies is wel'' re'oresente^ in 

 the collectiOTxS at hand. Despite the sn:a.ll size of the shell 

 the sculpture is ver: sharply chiseled. One nf the valves shows 

 traces of ri.'^r'k: coloration. 



Occurrence. - lower I.Iioeene : Borden be 'Is, Sowden, Jamaica. 



