- ■1Q7 - 



and broad, flat, almost smooth rauials "/hich are separated b^- 

 Tery narrov.' intt.rs paces. Externally it bears a ^iblj? elo^.e 



/ePiemb'l 'nrice to _. :,"' 1 1 "pxl-'^rin (l), a r.-iceios' -^^-nn the Jaloo.-a- 



fl). Trans. vVagner Free Inst. Sci., Philadelphia, vol. 1, ^o. Ifl, 

 pl. 16a, fig. 70, 1'^"?; Dall, i^^em" vol. 3, pt. 5, do. 10^0- 

 1001, 190:^. 



hatchie Pliocene. Ihe Bavden form., is however, j inflated 9nd 



.9.2 more tnxiid and hir-her umbones. In a:lcT\' ' _, e Pliocene 



species has a- more dcepl^^ ereniilated i: ' il ventral raar-^in and 



the sulci lerding up to the umbo are broader and deeper, but the 

 'ost conspiGuoiis flifffiren?e lies in the -nrpsenoe of an internal 

 rominent ''rib'', v.'hivTh ex'jsi'ids from ohe vii^oo almost to the level 



of the V.;. end of the posterior lateral tooth; this feature, 



'lioh ''•les been -._ ?? the '"'' ian'nnBtic character o'"'' th? cectTon 



-l:-rosteri£:ma Dall, 1^ eiitirel^^ lackirjg- in vaur/hani . 



Another, evexi more closely related ST)ecies, £. declive 



.--abb (2) from ''"he Go?'*'" ^ican '^■'i'^''e"nc , mailer, r-'ithout the 



{?.), Jour. Acad. Kat. 3ci., Philadelnhia, r r. , vol. n, p. .-^74, 



pl. 47, fig. 76, 1881. 



zngle between the postero-'"' ■'■^"■■1 ■■''■t'I -i-iO'^-tero-lateral mcr-^ns, r.l t'- 

 : more rounded postero-lateral niarr-in and. base, and more coiiGioic- 

 '-OUS internal radial lines; the sculpture is virtually the same. 



