LAMKU.IIiKAN- III 



. 



perfect material ran be o>mpaivd nt her dilferences will become apparent, especially 

 in their hinges and imi>ciilar impression-. the-e part- .ipp.Mrm- to be someu h.it 

 tiger in the Trenton shales species. 



fofm-itiim .m./ /<-<i/ify. Mldilli- tlilnl i.f : , ,1, Mimipaolft. 



Wiui i- i.i.\ 001 .v I'lrii-li. 



I'l.A 11 M.I. fiat. 3 and 1. 

 H'hitella con.-- .nth Ann. I; m.| N.il Minn.. |>. 247. 



Shell rat her beneath tin- nieliiiin si/e. obli-pie, ventricose. widest posteriorly, 



/iiiilal: Keaks lar^>, ]n)iiiiiit>nt. iiu-iirv>l; uniliones full, with .1 -harply rounded 

 ri-l^e or lint- of tfilil.oMty oxt'inlin^ lia'kwanl from the beaks to the posterior 

 extn-mity of the >ln>ll. I'anlinal ami posterior \o\yes slightly concave. Anterior 



short, narrowly roumh'il; ventral e'l^e very gently convex; posterior end pro- 

 and >barily ronii.lel in the lower half, more gently convex and sloping rapidly 

 forwanl above, nu'r^inu' gradually into the cm ve of tho dorsal side. Hinge line about 

 half as lon^ as the shell, with the edge inflected so as to form a narrow escutcheon, 

 extending luit little, if at all, in front of the beaks. Internal ligameutul supports 

 leave a distinct inipre^ion on each side of the postero-cardinal margin in casts of 



interior. Anterior nm-cnlar scars di-tinct though faintly impressed, situated in 

 the antero-dorsal angle. Surface of caste, especially in the lower and posterior parts, 

 marked with fairly distinct, rounded, concentric folds. 



The concentric undulations are stronger in this species than in any other known 

 to me. It is shorter than II'. /;<//)/</. more ventricose than II'. compressa, and has 

 much fuller iimhones than 1C. nliH.jidifn. In II'. wtfieldi the surface is not undulated, 

 the anterior end i- -uhungular alxtve. and the umbonal ridge sharper. 



Formation and loealitg. Miildl.- iliird <>f thi- Tr.-n(..n shales at MlDDeopolU, Minnesota. 



WHITEI.I.A IM-UAT -p. 



in. i. 



Shell subovate or oliscurely trapezoidal, moderately gibbous, with well shaped 

 and prominent umliones and strongly incurved beaks situated one-fourth of the 

 length of the shell behind the anterior extremity. I nilmnal ridge moderate, roui 

 except immediately behind tlir beak-. K-cutcheon nan> extending in front 



of the beaks. Surface of casts marked with numerou>. rather small, concei 

 furrows, which in parts may be (puite regular, but in otheis are thrown into bundles 

 so as to produce obscure undulations. Hinge unknown, muscular and pallia! 

 impressions^very faint. 



