640 GEOLOGICAL SURVEY OF IOWA. 



SURFACE marked by numerous concentric elevated bands, 

 which are pustulose or striato-punctate : entire surface, where 

 exfoliated, finely striato-punctate. 



This species is, I believe, usually referred to P. punctatus ; but the specimens 

 which I have examined have a proportionally longer hinge-line, and the surface is 

 less strongly marked, while the concentric bands are often equally or more con- 

 spicuous. 



Geological formation and localities. In the Keokuk limestone : Keokuk, 

 Iowa ; Warsaw and Nauvoo, Illinois. 



Orthis keokuk ( n. s.). 



PLATE XIX. FIG. 5 a, b. 

 Orthis umbraculum ? OWEN, Report on Wisconsin, Iowa and Minnesota, PI. v, f. 11. 



SHELL resupinate, somewhat broadly semielliptical in out- 

 line, depressed hemispheric : cardinal extremities rounded. 

 Ventral valve flat or slightly concave : area low, extending 

 to the hinge extremities; foramen forming an equilateral 

 triangle, closed by a pseudo-deltidium. Dorsal valve broadly 

 convex, the greatest elevation being a little above the mid- 

 dle, and often equal to one-third the width of the shell. 



SURFACE marked by even rounded radiating stride, which 

 increase by bifurcation and interstitial addition, and are 

 crossed by fine concentric striae. 



This species has been referred to O. umbraculum ; but a comparison with the 

 figures of that species given by DE KONINCK shows the cardinal area to be nearly 

 twice as large as in our specimens; the form is proportionally broader, the spaces 

 between the striae much greater than the striae (while in our shell they are less), and 

 the fine concentric markings, as shown in the enlarged figure, are very different from 

 those in the species under consideration. 



The figure of O. umbrarulum given in DUNKER & VON METER'S Palaeontographica, 

 Plate xxxviii, fig. 2, differs from that of DE KONIXCK, and from our own, in the 

 greater extension of the hinge-line; while the enlargement of strias differs equally 

 from the Belgian and American specimens. 



There is a closer resemblance between the present species and the figures of O. 

 undiferus ( Palaeontographica, Plate xliv, fig. 1 a, b, c) : the latter, however, more 

 nearly resembles, in its outline and general expression, the Orthis swallom of the 

 Burlington limestone. 



