PALEONTOLOGY OF IOWA. C37 



concentric wrinkles, which become more prominent towards 

 the cardinal margins, and are frequently nodulose at the 

 crossing of the radiating costae. Just below, and parallel to 

 the cardinal margin, the surface is marked by the bases of 

 four or five round spines on each side of the beak. In perfect 

 specimens, the entire surface is covered by fine concentric 

 stride. 



This species is a common form in the Keokuk limestone; differing sufficiently from 

 any of those in the same rock, and in the other limestones, to be readily recognized. 

 The P.Jlemingi, var. burlingtonensis, resembles this in some degree; but is propor- 

 tionally longer, less abruptly geniculate, and less distinctly marked by the mesial 

 sinus. That species also bears spines on the middle and front of the shell, and has 

 no range of spines on the hinge-line; while the species under consideration is con- 

 spicuously marked in this manner. 



Fig. 2 a. The interior of a dorsal valve, from which the ventral valve has been exfoliated. 

 Fig. 2 b. Profile view of the ventral valve. 



Fig. 2 c. Ventral valve of a large individual, which is somewhat broader than the pre- 

 vailing forms. 



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

 Iowa ; and Nauvoo, Illinois. 



Productus seiiiireticulatus. 



PLATE XIX. FIG. 4 a. 



Anomites semireticulatus : MARTIN, 1809. 



For synonymy and authorities, see DE KONINCK, Recherches sur les animaux fossiles, 83. 



DORSAL valve wider than long ; the cardinal extremities 

 auriculate, extended beyond the width of the shell below. 

 Visceral portion of the shell nearly flat, a little elevated in 

 the centre, with margin abruptly deflected both at the sides 

 and in front. 



SURFACE marked by strong radiating costse, which, on the 

 flat portion of the valve, are crossed by almost equally strong 

 concentric wrinkles : costae often nodulose at the junction 

 of the concentric wrinkles. 



This valve appears to be identical with P. semireticulatus in its broader and 

 stronger varieties, as figured by DE KONINCK ( Recherches sur les animaux fossiles). 

 The species, however, according to reliable authorities, assumes so many phases, that 

 we might include various other forms under the same designation; and it is difficult, 

 in a small collection of individuals, to determine the limits of its variation. 



