166 Trans. Acad. Set. of St. Louis. 



than the width of shell in front, and the margin of the car- 

 dinal area is angular and not rounded as in the casts. The 

 internal cast of the same individual is also preserved in the 

 same collection and does not differ from other similar speci- 

 mens. 



In authentic specimens of /S. peculiaris from the Chouteau 

 limestone of Cooper County, Missouri, the margin of the car- 

 dinal area is always rounded and the hinge-line shorter than 

 the Greatest width of the shell . These Chouteau specimens are 

 also, for the most part, internal casts, and they may have had 

 a better defined cardinal area in the shell itself, but the length 

 of the hinge could not have been greater than the greatest width 

 of the shell in any of the specimens which have been exam- 

 ined. If the extended hinge-line be a constant characteris- 

 tic of the Burlington specimens, they will have to be consid- 

 ered as distinct from JS. peculiarity, but sufficient material to 

 establish this distinction has not been available for study. 



The plications of the pedicle valve in the Chouteau lime- 

 stone specimens, are always better defined than in any of the 

 Burlington specimens, but this may be due to the different 

 sediments in which they are preserved. 



Reticularia cooperensis (Swallow). 



PL XIV. f. 14-15. 



Original description of Spirifer hirtus. ' ' Shell of medium 

 size, extremely ventricose, about once and a half as wide as 

 hio-h. Hinge-line very short, not more than one-third as long 

 as the width of the shell, front and cardinal angles regularly 

 rounded. Pedicle valve most ventricose a little forward of 

 the beak, which is obtuse and incurved; area scarcely percep- 

 tible ; delthyrium broad, triangular, nearly as wide at the base 

 as the length of the area; front half of the valve marked by 

 a broad, shallow, undefined sinus. Brachial valve less ventri- 

 cose than the opposite, regularly convex, without a visible 

 mesial elevation; beak obtuse, incurved, extending above the 

 cardinal line." 



" Surface marked by strong, equidistant, concentric ridges, 

 indicating different stages of growth; also by indistinct, 

 radiating striae, which form little pustules at the margin of 



