\ 



I >n n i- v. /- 



I' I V.I '.. -. 



ti I i) nun.: hlK'til i.-kn.-ss O..VJ 



n.: hlk'tit ii :' mm. 



Va! . .11^ -nine what in Outline, OB16 being oli-curely quadrate m- -iil>. 



other- -Inn t-renifiirm : dor.-al oiitliiii> more or less concave at the middle and 

 rounded or >iihan;,'iilar at the end-: ventral margin -tnni^'ly convex, (lie lower half 

 of the outliue beintf in some cases almost semicircular. A deep, suhceiitral. \ertical 

 sulcu- di\ -id. M info two larp- Milx<|iK(l loin--. These are very prominent, 



especially at their center-, and ris aliniptly from the flattened horders. At the base 

 an obscure nmne.-ii.in l>et\veen the loles may be noticed. 



When the \al\e- are not perfectly cleared from the matrix, some difficulty may 

 be experienced in distinguishing them from t be associated Ctenobolbinn <;ww, whicfc 

 also has a deep snlcu^. Hut in that -pecies the sulcus is curved and does not divide 

 the lobes ventral ly, ami the valves are longer and differently shaped. In fact the 

 two species ditler >o greatly that I cannot conceive how good specimens might be 

 confounded. 



uifi.m md locality.- i'|.|- r third <>r the Trenton shale8 (Phylloporlna bed), SU Paul, Minn.-wU. 

 U ' TKNnl'.nLHINA. 1'lrich. 



Ctno6o/6i'i. 1'i.iui ii, 1*0. J(.ur. Cln. Soc. Nat. Hist., rol. xlil, p. 108. 



:iiall, elongate-suboval, strongly convex, the posterior two-fifths more 



or less decidedly Imlhous or subglobular, and separated from the remainder by a 



deep, narrow and more or less oblique sulcus extending with a gentle curve from 



the dorsal margin more (ban half the distance across the valves toward the postero- 



ventral border. The anterior three-fifths often with another oblique but less 



impressed sulcus. Valves equal, the dorsal margin straight, hingement simple, the 



:al edge thick, and the true contact margins generally concealed, in a lateral 



frill" or fl. false border; surface granulose, smooth, or punctate. 



Type: /</<///) .-i7////i Kmmons sp. 



a 



c. 



I 



1 'tenolxilli. 

 emaciate I'lrich, >. -in - .-f i :n;ih, Illin..!-. This specie* proba' 



in the equivalent shale- .-.n Valley, Minn>>- 



48- 



