PALAEONTOLOGY OF IOWA. 639 



Productus setigerus ? var. keokuk. 



PLATE XIX. Fia. 4 b, c. 



SHELL somewhat ovoid, ventricose below, and tapering to- 

 wards the beak : hinge-line equaling or less than the width 

 of the shell below. Ventral valve flattened or broadly de- 

 pressed along the centre, with the umbo very prominent and 

 beak strongly incurved. 



SURFACE marked by numerous well-defined rounded costae, 

 which are about equal to the spaces between them. Costae 

 bifurcating on the upper part of the shell, and frequently 

 coalescing below ; marked at frequent intervals by the bases 

 of small tubular spines, which are crowded in great numbers 

 along the cardinal margins and the auriculate portions of the 

 valve. 



The specimen figured is somewhat distorted by pressure. Another specimen before 

 me, of similar character, is larger, with the umbo much elevated, and having a width 

 of two and a half inches at the base. In general form it resembles P. semireticulatiis, 

 and the specimens have been referred to that species : they differ, however, in the 

 more rapid diminution towards the beak, and the finer costae, as Well as the nume- 

 rous fine pustulose markings indicating the bases of.minute spines. In these respects, 

 there is a well-marked difference from authentic specimens of P. semireticulatus 



Having no means of verifying satisfactorily the ventral valve of P. setigerus, I 

 refer this, with doubt, as a variety of that species; the specimen figured ( Plate xix, 

 fig. 3 ) showing the spine-bases more elevated, and more nearly rectangular to the 

 surface than in these under consideration. 



Fig. 4 b. Cardinal view of a ventral valve. 



Fig. 4 c. Another view of the same specimen, looking apon the centre of the valve. 



Geological formation and localities. In the Keokuk limestone : Keokuk, 

 Iowa ; and Warsaw, Illinois. 



Productus \ittatus ( n. s.). 



SHELL subhemispherical or broadly semiovoid. Dorsal valve 

 moderately and uniformly concave. Ventral valve regularly 

 convex ; greatest elevation a little above the centre, flat- 

 tened or slightly depressed longitudinally along the middle : 

 umbo elevated, and beak abruptly and closely incurved over 

 the hinge-line. 



