SHUMARDELLA 221 



tion of a score or more of good examples of Meek and Worthen's species, 

 in comparison with Swallow's original definition of E. ringens, brings out 

 the fact that they are always very different in outline from the shell in- 

 dicated by Swallow. 'E. subtrigona is always notably broader than long, 

 while E. ringens is. distinctly defined as being longer than broad. The 

 species is a close ally of the rare R. isorhynctia M 'Coy, from the Mountain 

 Limestone fauna of Ireland, but the American species is much broader 

 than the Irish form with more conspicuously deflected anterior and antero- 

 lateral margins. The crest-like elevation of the antero-lateral margins 

 of the pedicle valve of C. subtrigona is also lacking in C. isorkyncha. 



The internal characters of this shell differ somewhat notably from those 

 of the genotype of Tetracamera. The characteristic buttress plates are 

 present in the pedicle valve, 'but the arrangement of the lamellag in the 

 rostral portion of the brachial valve is quite different. These differences 

 may be best comprehended through a comparison of the sections which 

 are given of the two species. It is possible that this species should 

 properly be recognized as a member of a distinct genus, but for the 

 present it seems best to consider it as congeneric with T. subcuneata. 



Horizon. Keokuk limestone. 



Genus SHUMARDELLA Weller 



Description. Shell rhynchonelliform, usually below medium size, sub- 

 ovate to subpentagonal in outline, often subglobular in form, the mesial 

 fold and sinus well developed anteriorly, the anterior margin of the sinus 

 much produced ; the surface marked by broad, low, rounded or subangu- 

 lar plications which are more strongly defined on the fold and sinus, and 

 which may be nearly obsolete. Internally the dental lamellae of the 

 pedicle valve are well developed. In the brachial valve a strong median 

 septum is present and is divided internally to form a narrow and deep 

 crural cavity which is closed on its cardinal side, this closure being con- 

 tinuous to beyond the point where the median septum is joined to the 

 hinge-plate, the lateral walls of the crural cavity being suspended in 

 the cavity of the valve for a short distance beyond the point where they 

 are connected with the median septum. The crura are formed by the an- 

 terior extension of the walls of the crural cavity where they are joined to 

 the hinge-plate. 



Remarks. This genus is characterized by the obsolescent plications of 

 the shell and by the crural cavity which is short in an antero-posterior 

 direction and entirely closed on its cardinal side. 



