498 GEOLOGICAL SURVEY OF IOWA. 



Dorsal valve deeply concave, following the curvature of the 

 opposite valve : a small space or area ? between the margins 

 of the two valves, along the hinge line. 



SURFACE of ventral valve marked by fine radiating striae 

 and a few concentric imbricating lines of growth, which 

 become wrinkles on the cardino - lateral margins : a few 

 scattered nodes, or bases of tubular spines mark the surface 

 at irregular intervals. Dorsal valve marked by regular con- 

 centric imbricating lines, without visible radiating striae. 



This species is proportionally narrower than any of the Productidese (which I 

 know) in the rocks of this age, and the strongly marked difference in the character 

 and direction of the striae on the two valves is a very characteristic feature of the 

 shell. In general form and the character of the striae of the ventral valve, it bears 

 some resemblance to P. undiferus of DE KONINCK, but the concentric striae of the 

 dorsal valve are quite distinctive. 



Fig. 7 a, b. Dorsal and ventral views of the. shell. 



Fig. 7 c. Profile of the same. 



Fig. 7 d. Enlargement of the striae from the surface of the ventral valve. 



Fig. 7 e. Enlargement of concentric striae from the surface of the dorsal valve. 



Geological formation and locality. In calcareous shales of the age of the 

 Hamilton group : Lime creek above Kockford, Iowa. 



Productus pyxidatus ( n. s.). 



PLATE III. FIG. 8 a - e. 



SHELL semielliptical, breadth greater than the height ; 

 cardinal extremities somewhat rounded, the hinge line being 

 shorter than the greatest width of the shell. Ventral valve 

 broadly convex, somewhat flattened and a little recurved at 

 the cardinal extremities, gibbous on the umbo and beak re- 

 curved. Dorsal valve broadly concave, a little flattened at 

 the cardinal extremities ; concavity less than the convexity 

 of the opposite valve. 



SURFACE marked by close concentric striae, and more con- 

 spicuous imbricating lines of growth : the ventral valve 

 sometimes having elongated spiniferous tubercles or ridges, 

 which are arranged in alternating series ; the dorsal valve 

 sometimes with indistinct radiating ridges, which are not 

 spiniferous. 



