PALAEONTOLOGY OF IOWA. 



511 



different stages of development and different degrees of preservation. The species 

 assumes various phases, from the extremely gibbous form with short hinge extremi- 

 ties as in fig. If, g, to the most extravagant forms produced by extension of the 

 hinge line, sometimes upon one and sometimes upon both sides. The surface is often 

 marked by ten, twelve or fourteen strong plications, beyond which there is a distinct 

 winglike extension on which the folds are much more slender; and when this exten- 

 sion is not so well marked, there is the same change from the larger to the smaller 

 plications. The concentric lamellae are sometimes quite conspicuous, and the radiating 

 striae subdued or worn so as to be scarcely visible; while in the other extreme, the 

 radiating striae, with granulose elevations, cover the concentric striae, and sometimes 

 both are equally preserved. The granulose points are doubtless the bases of slender 

 spines or setae, which originated at the edge of the laminae. The following are illu- 

 strations of a few of the phases of form, which might be much increased without 

 exhausting the variety. 



Fig. 1 a. Dorsal valve of a young specimen of this species. 

 Fig. 1 b. Cardinal view of a larger individual. 



Fig. 1 c, d. Dorsal and ventral views of a large individual, in which one side is much 

 more extended than the other. 



Fig. 1 e. Cardinal view of the same, showing the extension of the area to tlje right" where 

 the shell is broken off. The shell has probably extended an inch beyond the 

 present termination. 



Fig. 1 /, g. Cardinal and dorsal views of a more gibbous specimen, where the extremities 

 are much less extended. 



Fig. 1 h. Profile view of the preceding specimen. 



Fig. 1 i. Enlargement of the surface, showing the radiating and concentric striae, which 



presents only one of several characters of surface of the specimens, in various 



degrees of perfection or conditions of preservation. 



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

 the Hamilton group : New-Buffalo and Independence, Iowa ; and Rock 

 island, Illinois. 



Spinier marionensis. 



PLATE VI. FIG. 1 a, b, c. 

 Spirifer marionensis : SHUMARD, Geol. Keport of Missouri, 1855, pa. 203, pi. C, f. 8. 



SHELL nearly semicircular, moderately gibbous, length 

 about equal to two-thirds the greatest width ; valves almost 

 equally convex ; hinge line extended into acute mucronate 

 points. Ventral valve broadly semielliptical or semicircular, 

 greatest convexity above the middle ; umbo gibbous ; beak 

 pointed and incurved ; sinus well defined, narrow and an- 

 gular at the beak, becoming broad and shallow below, and 

 marked by two or three dichotomizing plications : area 

 narrow ; sides nearly parallel and extending to the hinge 



