636 GEOLOGICAL SUKVEY OF IOWA. 



SURFACE marked by even, rounded, prominent strise, which 

 bifurcate two or three times on the upper half of the valve, 

 and rarely below the middle : radiating striae crossed on the 

 upper half of the shell by concentric folds or wrinkles, 

 which become more conspicuous on the margins. Entire 

 surface marked by fine concentric striae : radiating striae 

 sometimes interrupted or pustulose on the upper part of the 

 shell, and, on the marginal band, distinctly showing the 

 bases of small round spines. 



This shell resembles the P. marginicinctus of Dr. PROUT, but differs in form, being 

 more ventricose, wider towards the beaks, and less depressed down the centre; the 

 radiating folds or striaa are finer, more even and continuous, and there are no spine- 

 bases on the middle of the shell. There are likewise other less conspicuous differences 

 in the two species. 



Fig. 1 a. Profile view of a specimen. Fig. 1 b. Ventral valve of the same. 



Geological formation and locality. In the Kcokuk limestone : Nauvoo, 

 Illinois. 



Productus mesialis ( n. s.). 



PLATE XIX. FIG. 2 a, b, c. 



SHELL varying in form from length and breadth equal, to 

 width much greater than length : hinge-line equalling the 

 width of the shell, subauriculate. Ventral valve extremely 

 ventricose, abruptly curving or geniculate above the middle 

 of the valve, marked by a broad longitudinal sinus which 

 reaches nearly to the beak of the shell, and becomes more 

 conspicuous at and below the geniculation. Dorsal valve 

 concave in a small space just below the beak, and nearly flat 

 to the geniculation of the valve, where it is bent almost 

 rectangularly upwards ; having a longitudinal mesial eleva- 

 tion on the lower half of the valve. 



SURFACE marked by elevated, rather sharply rounded costae, 

 which bifurcate two or three times on the upper half of the 

 shell, and sometimes below the geniculation ; becoming gra- 

 dually less prominent on the lower half of the shell. On the 

 upper part of the shell, the surface is marked by strong 



