PALJEONOLTOGY OF IOWA. 503 



simple rounded plications on each side, which are larger than 

 those in the mesial sinus and elevation ; these gradually 

 diminish in size, till they become extremely slender on the 

 extremities of the shell : depressions between the plications 

 narrower than the plications themselves, except those bor- 

 dering the mesial fold, which are broader and deeper than 

 the others. Entire surface marked by extremely fine radiating 

 striae, which cover alike the plications and the spaces between 

 them : these sometimes give the plications a grooved ap- 

 pearance under a lens. Equally slender concentric undu- 

 lating striae mark the whole surface; and along these are 

 arranged minute granulations, which are only visible under 

 a lens : a few more distinct imbricating lines of growth are 

 sometimes seen. 



This species bears a close resemblance to Spirifer archiaci of MURCHISON, as 

 described and figured in the Geology of Russia and the Ural mountains, Vol. ii, pa. 

 155, pi. iv, f. 5 a, b, c ; and by DE KoffusrcK, Descriptions des Animaux fossiles de 

 Belgique, pa. 254, pi. xiv, f. 5; but our specimens do not show the variety of form 

 observed in the European species. 



A comparison of several specimens shows that the plications, as well as the mesial 

 sinus and elevation, are more angular, and the foramen narrower in the European 

 than in the American specimens, while the former still retain some remains of fine 

 radiating striae. In the American specimens the plications are directed towards the 

 beak, a few of the outer ones only running out upon the hinge line ; while in the 

 European specimens a larger number of plications terminate on the hinge line before 

 reaching the beak. These differences, which are perhaps too slight to be generally 

 regarded as deserving of specific distinction, are, nevertheless, of importance when 

 regarded in connection with the wide geographical separation of the Russian, Belgian 

 and Iowa specimens, and the absence of knowledge regarding the exact parallelism 

 of the deposits in the two countries. 



Fig. 2 a, b. Ventral and dorsal views of a specimen of medium size. 



Fig. 2 c. Cardinal view of the same. 



Fig. 2 e. Enlargement, showing fine striate and granulose surface. 



Geological formation and locality. In calcareous shales of the age of the 

 Hamilton group : Lime creek above Rockford, Iowa. 



