516 GEOLOGICAL SURVEY OF IOWA. 



and finally closely bent over the beak of the opposite valve : 

 a narrow false area is sometimes observable. Shell broadly 

 and deeply sinuate in front. 



The specimens from Iowa, in their young and half-grown condition, are usually 

 more finely striated than those from the Hamilton group in New-York. In older 

 specimens the difference in this respect is scarcely observable, though there is always 

 a difference in the proportions and general expression of the shell. 



Our specimens resemble very closely the figures of Terebratula zonata ( DUXKER 

 and VON METER'S Palseontographica, pi. 24, f. 6). 



Fig. 4 a, b. Dorsal and ventral views of a small specimen. 



Fig. 4 c. Ventral view of a larger individual. 



Fig. 5 a, b, c. Dorsal, ventral and profile views of a very large individual. 



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

 the Hamilton group : New-Buffalo, Independence, Lime creek and other 

 places in Iowa ; Rock-island, Illinois. 



Euomphalus cyclostomns (n.s.). 



PLATE VI. FIG. 6 a, b, c. 



SHELL discoid, depressed convex above, broadly and deeply 

 umbilicate below ; volutions about five or six, gradually in- 

 creasing from the apex, regularly rounded; aperture circular; 

 umbilicus broader than the mouth. 



SURFACE marked by fine strise of growth, which, from in- 

 terruption in growth, are arranged in fascicles or wrinkles. 



This species is common in several localities of the rocks of the age of the Hamil- 

 ton group, and is, I presume, the same species mentioned in Dr. D. D. OWEN'S Report, 

 but I am not aware that it has ever been described. 



Fig. 6 a. View of the summit of a specimen of the ordinary size. 



Fig. 6 b. View of base of the same. 



Fig. 6 c. Profile view, showing the elevation of the spire. 



Geological formation and localities. In calcareous shale and in a ferrugi- 

 nous limestone of the age of the Hamilton group : Pine creek, Fairport and 

 New-Buffalo, Iowa. 



