PALAEONTOLOGY OF IOWA. 635 



BRACHIOPODA OF THE KEOKUK LIMESTONE. 



X 



THE following species are among the most prominent and 

 characteristic of the Keokuk limestone, in those localities 

 which I have examined. Numerous other species occur in the 

 same rock, some of which are doubtless as widely distributed, 

 and as characteristic of the formation, as any here given. In 

 making the selection, I have endeavored to give representa- 

 tive species of the same genera as those of the preceding 

 and following formations, for the purpose of furnishing the 

 means of instituting a comparison between the most nearly 

 allied species of the different rocks. The range of my il- 

 lustration, however, has been mainly limited to certain parts 

 of Iowa and the adjacent portions of Illinois ; not feeling 

 at liberty to introduce here other species which I know from 

 the same horizon in Missouri, but which nevertheless Offer 

 quite as great a distinction as those here illustrated. 



Productus woitheni ( n. s.). 



PLATE XIX. FIG. 1 a, b. 



SHELL varying from small to medium size, as wide or wider 

 than long : hinge-line equalling the width of the shell, au- 

 riculate. Ventral valve extremely arcuate, very ventricose, 

 flattened in the middle : margin spreading abruptly and 

 again abruptly inflected, leaving a prominent rim or cingu 

 lum, which is less conspicuous in front. Dorsal valve concave 

 in the middle near the beak, elevated towards the margins, 

 and flattened at the cardinal extremities ; the middle and 

 lower part of the valve nearly flat, or moderately concave. 



