PALEONTOLOGY OF IOWA. 707 



and moderately elevated. Ventral valve very regularly 

 convex, arching from the beak to base ; extremities com- 

 pressed ; beak arching and incurved ; sinus simple at the 

 beak, and marked by three plications which are strongly 

 developed on the lower half of the shell ; area narrow, well 

 defined, and extending to the hinge extremities : foramen 

 large, triangular, the upper part hidden by the incurving 

 beak, and sometimes partially closed above by the thickening 

 of the dental lamellae. 



SURFACE marked by from sixteen to eighteen rounded 

 plications on each side ; those adjacent to the mesial lobe 

 and sinus bifurcating, and one division on each side often 

 depressed so as to give the appearance of fine plications in 

 the sinus ; the others simple : a few strong undulating and 

 imbricating lines of growth, with fine striae between ; and 

 the entire surface, in well-preserved specimens, marked by 

 fine radiating striae, with a reticulate ornamentation visible 

 only under a lens. 



This species has sometimes been referred to Spirifer incrassatus, EICHWALD; but 

 it differs essentially from the figures and descriptions of that species, in being always 

 angular instead of rounded at the cardinal extremities. The mesial fold in our shell 

 is smaller, and marked by plications smaller and less distinct than those on each 

 side; while in S. incrassatus, these are represented as larger than those on other parts 

 of the shell. There are from fourteen to sixteen plications on each side in full-grown 

 shells of the present species, while the number on the same parts of S. incrassatus is 

 given by MUKCHISON and DB VERNEUIL as twelve. These authors, though mentioning 

 the concentric striae, do not speak of the fine radiating striae, which, in our shell, are 

 more conspicuous than the others. 



Fig. 6 a. Dorsal view of an individual of medium size, with the cardinal extremities 

 somewhat mucronate. 



Fig. 6 b. Ventral view of the same. 

 Fig. 6 c. Profile of the same. 



Fig. 6 d, e & /. Dorsal, ventral and profile views of an older shell, which is more gib- 

 bous, and has the cardinal extremities less extended. 



Fig. 6 g-. Interior of an imperfect ventral valve, showing the foramen and muscular 



impressions. 

 Fig. 6 h. Interior of the ventral valve of an older specimen, which is more gibbous and 



the beak more arched. 



Fig. 6 f. Enlargement of the surface, showing the fine radiating and concentric stria3. 



Geological formation and localities. In the Kaskaskja limcstppe : Kas- 

 kaskia, Chester, and other places in Illinois, 



