226 MISSISSIPPIAN BRACHIOPODA 



over by the joined portion of the hinge-plate and open only in an anterior 

 direction, the support of the hinge-plate by the septum is discontinued 

 before the line of articulation is reached and from this point forward 

 each lateral division of the hinge-plate is unsupported, anteriorly each 

 division is produced into the cavity of the valve as one of the crura. 



Surface of both valves marked by fine, rather regular concentric 

 stria? and by occasional strong lines of growth. 



Remarks. Shumard illustrated two. different individuals with his 

 original description of Rhynchonella missouriensis, fig. 5a being a smaller 

 and much smoother shell than figures 5b-c. The original description was 

 evidently made from the larger and more strongly plicated shell, and 

 that is here taken as the typical form of the species. Both of the illus- 

 trated forms are commonly represented in collections of the Chouteau 

 limestone fauna from central Missouri, and while an imperfect gradation 

 from one form to the other can be traced, the two forms are easily distinct 

 enough to be considered as different species. In his lists of species 

 Shumard records R. obscura-plicata, a name by which he undoubtedly 

 designated the shell here described, although he never defined it. and 

 the name by which Missouri collectors have known it since that time. 

 In 1860 Hall described Rhynchonella obsolens, without illustrations, from 

 the goniatite-bearing Rockford limestone of Indiana, and his description 

 agrees fairly well with examples of this smooth form from the Chouteau 

 limestone of Missouri. In 1866 Meek and Worthen described and illus- 

 trated as R. missouriensis a shell from the same Rockford limestone which 

 agrees satisfactorily with the smooth form of that shell as illustrated by 

 Shumard, and it is more than probable that the observers had rep- 

 resentatives of the same shell as that described by Hall. Under these 

 circumstances, therefore, if \ve restrict the use of the name missouriensis 

 to the larger shell of Shumard, either Hall's name or Shumard 's obscura- 

 plicata may be available for the form here described, and the latter of 

 these must be ruled out because it was never properly defined. 



In their generic characters, as shown by serial sectioning of the shells, 

 R. missouriensis and R. obsolens are identical, but they cannot be included 

 in any of the recognized generic divisions of rhychonelloid shells and have 

 been made to constitute the new genus Shumardella. S. obsolens may be 

 distinguished from 8. missouriensis by its smaller size, its more globose 

 and less angular form, its much smoother and less strongly plicated 

 shell, and by the more distinctly emarginate anterior outline. 



The species has some external resemblance to members of the genus 

 Liorhynchus and has sometimes been incorrectly identified as L. boonen- 

 sis, a rare species which was originally described from the Burlington 

 limestone. 



Horizon. Chouteau limestone. 



