RETICULARIA 431 



dinal extremities ; mesial fold usually obscure or obsolete except near the 

 front margin where it is low and ill-defined, but sometimes more prom- 

 inent and originating in the umbonal region; the beak short, incurved, 

 the umbonal region projecting beyond the cardinal margin; cardinal 

 area narrow, lying nearly in the plane of the valve. Internally the 

 cardinal process is scarcely more than a small, concave, differentiated 

 region of the inner surface of the valve just under the beak, marked by 

 fine vertical ribs and furrows, anteriorly from the cardinal process a 

 median septum of greater or less strength extends one-third or more of 

 the length of the valve, and on either side the cardinal process is flanked 

 by strong crural plates which are joined to the median septum; the 

 muscular scars are poorly defined ; the entire internal surface covered by 

 fine radiating costse similar to those of the opposite valve. 



Surface of both valves marked by concentric bands of crowded, regu- 

 larly arranged, double-tubed, oblique spines, the successive rows of 

 which are imbricating in arrangement, the bases of the spines are con- 

 tinued posteriorly to the next concentric band as a slightly raised ridge, 

 giving to the surface of the shell when slightly worn the appearance of 

 having both concentric and radiating markings, with the radiating 

 markings much finer than the concentric ones. The radiating costse 

 of the interior of the valve are not infrequently clearly visible upon 

 partially exfoliated specimens. 



Remarks. Hall's original specimen of this species is recorded from 

 the Keokuk limestone of Keokuk, Iowa. It differs from most of the 

 examples from that locality in having a more strongly developed mesial 

 fold and sinus, although occasional specimens are found which have 

 this character even more strongly marked than in the type. The type 

 specimen is not in a condition to show the internal radiating costae of the 

 species but this character is well developed in all specimens preserved 

 in such a condition as to exhibit it, and in internal casts from the chert 

 it is especially well shown. The species differs from R. setigera from the 

 higher Mississippian beds, in the strong development of these internal 

 radiating costse, in the lesser development of the fold and sinus 

 and in the finer rows of concentric spines. The strong development of 

 the internal markings seems to be, perhaps, the most characteristic 

 feature of the species. 



Horizon. Keokuk limestone. 



RETICULARIA SETIGERA (Hall) 

 Plate LXXIV, Figs. 12-22 



1858. Spirifer setigerus Hall, Geol. Iowa, vol. 1, pt. 2, p. 705, pi. 27, 



figs. 4a-b. 

 1877. Spirifera setigera Hall and Whitfield, U. S. Geol. Expl. 40th Par., 



vol. 4, p. 270, pi. 5, figs. 17-18. 



