SHUMARDELLA 



513 



dian septum is soon disconnected from the cardinal side of the valve (fig- 

 ure 14e), and continues with gradually decreasing height well toward 

 the middle of the valve. With the disconnection of the median septum 

 cardinally, the two plates forming the sides of the inclosed crural cavity 

 continue as free plates extending into the cavity of the valve (figure 14e), 



^ & <^ 



Q 



(D 



8 8 8 



5^ 



Q 



Figure 14. — Cross-sections of the rostral Portion of Shumardella missouriensis {Shum.) 



This series of nine cross-sections (X -V2) is from a specimen from the Chouteau lime- 

 stone of Missouri 



the space between the plates is arched over cardinally for a time, but with 

 the reduction of the width of these vertical plates the hinge-plate becomes 

 divided just posterior to the articulation of the valves (figure l-i/). 

 Beyond the articulation of the valves the inner margins of the divided 

 hinge-plate are produced anteriorly into the bases of the crura. 



SHUMARDELLA 0BS0LESCEN8 (HALL) 



Associated with S. missouriensis in the Chouteau limestone is a smaller, 

 more subglobular shell, with nearly obsolete plications, which has com- 



' ^. A & & 



Figure 15. — Oross-sections of the .ral Portion of Shumardella ohsolescens t,Hall) 



This series of eight cross-sections ( X 2 ^^ ) is from a specimen from the Chouteau lime- 

 stone of Missouri 

 XXXVI — Bull. Geol. Soc. Am., Vol. 21, 1909 



