704 GEOLOGICAL SURVEY OF IOWA. 



Retzia vera ( n. s.). 



PLATE XXVII. FIG. 3 a. 



SHELL ovate in outline, broadly rounded at the base, some- 

 what compressed in the lower part, regularly and equally 

 swelling above, somewhat gibbous on the umbones : valves 

 equally convex ; hinge-line straight, auriculate at the ex- 

 tremities. Dorsal valve regularly convex : umbo prominent. 

 Ventral valve somewhat arcuate, a little flattened on the 

 middle in old shells ; sides tapering to the beak, which is 

 strongly arched, and truncated at the extremity by a round 

 foramen : cardinal extremities slightly produced to meet 

 those of the opposite, leaving a narrow area between the 

 two valves. 



SURFACE marked by from forty to fifty simple rounded 

 radiating costse, which are fine and scarcely perceptible to 

 the naked eye, but become much coarser as they extend. A 

 small extent on either side below the beak, particularly on 

 the ventral valve, is free from plications. Radiating costae 

 marked by fine closely arranged concentric striae, which are 

 only visible under a lens : minute structure punctate. 



This shell resembles the Retzia ( TerebratuLa ) marcyi of SHUMAKD ( MARCY'S 

 Report of an Exploration of the Red River of Louisiana, pa. 204, pi. 1, f. 4 a, b ), 

 but differs in its larger size and greater number of striae; and judging from the as- 

 sociation with T. subtilita, the R. marcyi is of the age of the Coal measures. 



The species under consideration differs from R. verneuilana of the Warsaw lime- 

 stone in its larger size and coarser costae, and in having its greatest width below the 

 middle of the shell, and the beak more strongly arcuate. 



Fig. 3 a. Ventral view of a specimen, each valve of which is marked by fifty costse : 

 a much smaller specimen has forty. 



Geological formation and locality. In the Kaskaskia limestone : Chester, 

 Illinois. 



Retzia vera, var. costata. 



PLATE XXVII. FIG. 3 a, b. 



The specimen figured is much larger than the preceding, having the same general 

 form, or being a little more flattened upon the back. The surface is marked by thirty- 

 eight simple rounded costse : entire surface beautifully punctate. 



