514 S. WELLER MISSISSirPIAN RHYNCHONEIiLIFORM SHELLS 



monly been considered as a variet}^ of S. missouriensis. This shell is 

 perhaps identical with Rhynchonella ohsolescens Hall, from the Eockford 

 limestone of Indiana ; at least the external form is essentially the same, 

 and it may be designated as 8. ohsolescens until the internal characters 

 of the Eockford shells are investigated. A series of cross-sections of this 

 shell is shown in figure 15, which shows that in its essential features it 

 agrees with S. missouriensis and may be included in the same genus. 



Ehyn-ohopora King 



GENERAL CBARACTERISTICS 



The genus Rhynchopora has been established to include several rhyn- 

 chonelloid shells which are known to possess a punctate shell structure, 

 this shell structure being the essential character upon which the genus 

 has been based. The internal structure of the shell has never been de- 

 scribed, but on investigation several species seem to exhibit additional 

 characters of generic importance. No opportunity has been afforded to 

 examine the genotype of the genus. 



RHYNCHOPORA PU8TUL0SA (WHITE) 



This species occurs in one of the Kinderhook formations at Burlington, 

 Iowa, and a series of cross-sections of a typical representative of the spe- 

 cies is shown in figure 16. After the second section the pedicle valve is 



OQ 



®c <z>. <S>e KSy^ xjy^ 



xr/^ ':^^ K^j 



Figure 16. — Cross-sections of the rostral Portion of Rhynchopora piistiilosa. (White) 

 This series of ten cross-sections (X 2 Mj ) is from a specimen from the Kinderhook bed 



No. 4, at Burlington, Iowa 



not shown in these figures, as it possesses no features of diagnostic value. 

 In the brachial valve a well defined median septum is present which is 

 divided to form a crural cavity as it approaches the cardinal side of the 

 valve (figures 16c, d, e, /), but, unlike Camarotoechia, this crural cavity 

 is roofed over by the undivided hinge-plate. Even after the median 

 septum has become disconnected cardinally the undivided hinge-plate 

 persists nearly to the point where the crusal bases become free (figures 

 16^, ti, i). This character of the undivided hinge-plate is entirely dif- 

 ferent from any other generic group of Mississippian rhynchonelloids 



