494 GEOLOGICAL SURVEY OF IOWA. 



a few strong crenulations upon the inner edge of the area : 

 cardinal process small ; muscular impressions subreniform, 

 with three elevated oval spaces near the centre of the valve. 

 Entire inner surface papillose. 



This species bears much external resemblance to 5. nacrea of the Hamilton group 

 of New- York, but the internal structure is very different. 



Fig. 3 a. Exterior view of a ventral valve of large size. 



Fig. 3 b. Interior of the ventral valve, showing the muscular and vascular impressions. 



Fig. 3 c. Interior of a dorsal valve, showing the crenulated hinge line and muscular 

 impressions. 



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

 Hamilton group : Rock-island, Illinois. 



Stropliodonta reversa (n. s.). 



PLATE III. FIG. 4 a - d. 



SHELL resupinate, semicircular or semielliptical, width 

 greater than the length; hinge line extended beyond the 

 width of the shell below. Ventral valve slightly convex in 

 the middle towards the beak, flattened along the hinge and 

 concave below, the shell being abruptly deflected near the 

 margin. Dorsal valve flat on the umbo and slightly concave 

 near the beak, abruptly bent downwards or almost geniculate 

 below the centre : area common to the two valves, that of 

 the ventral valve a little higher towards the middle ; fora- 

 men none. 



SURFACE strongly striated : striae simple, strong and an- 

 gular near the beak, and dichotomizing once, twice, or thrice 

 before reaching the centre of the shell, increasing also by 

 implantation. Entire surface punctate. 



Interior of ventral valve marked by small reniform mus- 

 cular impressions, which are sometimes not distinctly limited : 

 vascular impressions large flabelliform or double ovate areas, 

 which in the upper part are defined on their margins by a 

 thickening of the shelly matter, while ramifying linear 

 impressions are continued to the margin of the valve : car- 

 dinal process prominent, bilobed, with a deep depression 



