

LAWi HI vi \ 



in the mi.l.||i>: po.termr end l-roadly rounded, -dighth pn,d<, i more Ktrongly 



convex in the lower half, tin- upper more yi-ntly curvd an. I -oni-times fon 



obtt^yugnlarjttDctioii with the hinge I. !.e-eventh of the en 



lengtli nf -hell Mund the anterior extremi' ,-r small. incurved. proje, 



...... leniti-ly al.ove tln> hinge; umliones compressed in the cast, a little less *. 



-hell. Sin-fact- moderately .-.uv.-x. mo,t prominent 'lie unil>ODal ridge, thp 



latter n little stronger than UMUI! for species of this gemw. Cardinal -lope concave. 

 A l.roa.l ami .-..nipanitively well-hMin.l in.'-ial .Irpression extends obliquely acron 

 th.--li.-ll from the Leak and, expanding, caoaeH the tttraigbteninR of the ventral mar- 

 Kin. Kxivi-tii.;: in this part the shell is very thin, and the anterior muscular 

 which i> c(ini[aratively <.f small n .-arcely distinjrui.shahle in casts. Surface 



r.ithrr Mi,.,-iirely markel with nninen.u- tine concentric lines and a few stronger 

 varices of growth. 



A- might lie e\pecte<l. thi- early specie^ exhibits features intermediate between 

 tho-e marking the group of forms which I now propose to distinguish as A 



ami true .l/.//../ny,\. This is seen in the thin shell and consequent indistinct.nett 

 <if the anterior ail<lm-tnr impression, in the full and prominent umbones and in 

 the convex rather than straight or concave basal line. At first I was inclined t<> 

 put the -pecie* into the new genus, but later comparisons have shown that Arlitto 

 MMprwas at that time already well established and that M. similia belongs to the line 

 which finally produced M. liolarii. Then the comparatively strong mesial de| 

 -ion indicates .lA"//oA,vi's and not Artinm, - 



Compared with Minnesota Trenton species, all the others referred to Modiolopgu 

 narrower postericrrly. The At4inmya niperba Hall, sp., has a larger anterior It'* / 

 end. the postero-basal margin more produced, and the umbones larger. The 



nmle-cril-ed Kentucky -prcie- referred to in the i proves to be 



ly related to C. ntbovatn I'lrich. 



, .i./ /.HI/I/. -MiiMI- tlnr.l of the 'I ,alc ml MlomvpnlU, MlnonoU. 



Ml MS, f. *p. 

 ri.ATE XI-H. Klii!' I? and ML 



This ,11,'H i- -o much likr ' 'u7i.v that at tir-t I believed it might bel- 

 to the same species. C'arefully comparel. however, it proved to differ in several 

 characters that are more important than striking. The umlxmee are larger 

 and very little compressed, and the mesial snlcna, which is a well marked 

 featur- in that tpede* i- MU -liable.. The outline also is a li? 



ditlerent. the posterior bight, being i y somewhat less than in the preced- 



ing >pr. 



