676 GEOLOGICAL SURVEY OF IOWA. 



ly and extremely below the middle and towards the base. A 

 few rounded prominent spines occur on the central portion 

 of the ventral valve, at the coalescing or bifurcation of the 

 costas. The upper part of the shell is marked by concentric 

 wrinkles, which become more conspicuous on the auriculate 

 cardinal extremities, and are there marked by a few tubercles 

 which are probably the bases of spines. 



This species possesses some characters in common with P. altonensis of NORWOOD 

 and PRATTEN; but the spreading and subplicated character of the front of the shell, 

 as well as the absence of the reticulated appearance mentioned by these authors, 

 prove it to be quite distinct. In general form and aspect it resembles the P.Jlemingi, 

 var. burlingtonensis ; but is more finely costate, the spines are smaller, and the mesial 

 depression not existing or inconstant. 



It also resembles the figures of P.flexistrice of M'Cor; but is more finely costate, 

 and bears spines on the anterior portion of the shell, which are not shown in the 

 figures of M'CoT or DE KONINCK. 



Fig. 2 o. Ventral valve of this species. [ The anterior longitudinal folds or undulations 

 are not well shown in the figure.] 



Fig. 2 b. Profile view, showing the extremely arcuate character of the shell. 

 Fig. 2 c. View of the cardinal and upper portion of a ventral valve. 

 Fig. 2 d. Dorsal valve of this species. 



Geological position and locality. In the St. Louis limestone, associated 

 with Lit host ration mamillare and Productus mar ginicinctus : Milan, 111. 



Spirifer keokuk, var. 



PLATE XXIV. FIG. 4 a, 6, d. 



SHELL transversely semielliptical ; hinge-line longer or 

 shorter than the width of the shell below : valves nearly 

 equally convex. Dorsal valve with the mesial sinus abruptly 

 elevated, simple above, and becoming divided into four 

 distinct plications below. Ventral valve with the sinus simple 

 at its origin, and becoming marked by a strong flattened 

 plication in the bottom, with a smaller elevated one on each 

 side : beak closely incurved, linear ; foramen broad. 



SURFACE, on each side of the mesial fold and sinus, marked 

 by from seven to ten simple plications ; the two adjacent to 

 the mesial sinus and elevation originating from a single one 

 near the apex. Entire surface cancellated by extremely fine 



