PALEONTOLOGY OF IOWA. 497 



SURFACE covered by slender closely arranged radiating 

 striae, crossed by finer elevated concentric lines which be- 

 come more prominent on the radiating striae of the ventral 

 valve, giving a minutely nodulose or granulose character to 

 the entire surface when well preserved. The radiating striae 

 of the dorsal valve are more evenly rounded, and increase 

 mainly by bifurcation ; while those of the opposite valve 

 increase by interstial additions ( this character may be sub- 

 ject to variation), and are crossed by even, raised, concentric 

 lines which produce no granulations on the surface. 



Internally the ventral valve shows a very small, scarcely 

 conspicuous central cardinal process. The muscular impres- 

 sions are usually slightly marked, and the vascular impres- 

 sions double, foliate or flabellate, with a scarcely defined 

 separating ridge, and margined by a slight elevation of the 

 lamellae proceeding from the hinge line. 



This species may be recognized by its extreme tenuity and the narrow space 

 between the two valves, which scarcely exceeds the thickness of the shell. The area 

 presents no appearance of a triangular foramen or a deltidium, though there is 

 sometimes a slight sinuosity in the margin. In the New- York specimens, the central 

 cardinal or dental process of the ventral valve is conspicuous, while in the Iowa 

 specimens it is very subdued. The Iowa specimens occur chiefly as single valves, the 

 ventral valve only being usually preserved, while rarely there occur specimens re- 

 taining the two valves. 



Fig. 6 a. View of the ventral valve. 



Fig. 6 b. Interior of ventral valve, showing the foliate vascular impressions, entire 



area, etc. 

 Fig. 6 c. Enlargement of the striae. 



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

 the Hamilton group of New-York : New-Buffalo and Independence in 

 Iowa, and Rock-island in Illinois. 



Productus dissimilis (n. s.). 



PLATE III. FIG. 7 a, b, c, d e. 



SHELL semielliptical in outline ; length and breadth nearly 

 equal. Ventral valve extremely convex, 'gibbous in the mid- 

 dle and towards the beak ; umbo elevated above the hinge 

 line ; beak incurved ; hinge extremities slightly auriculate. 

 [ IOWA SURVEY.] 63 



