522 GEOLOGICAL SURVEY OF IOWA. 



LAMELLIBRAflCIIIATA OF THE CHEMUNG GROUP. 

 Avicula circulus. 



PLATE VII. FIG. 9. 

 Avicula circulus : SHUMARD, Geol. Report of Missouri, 1855, pa. 206, pi. C, f. 14 a, b. 



SHELL suborbicular, slightly convex ; .cardinal line less 

 than two-thirds the width of the shell ; length and width 

 nearly equal ; beak small, pointed and slightly elevated 

 above the cardinal line ; ears small, acute ; posterior slope 

 a little depressed ; basal and baso-lateral margins regularly 

 rounded; posterior side, from hinge extremity, somewhat 

 straight. 



SURFACE marked by fine, closely arranged, concentric striae, 

 and equally fine radiating striae. 



The specimens examined are not entire, but the hinge line sometimes appears to 

 be lesb than the proportion given above. 



Fig. 9. The right valve of this species. 



Geological formation and locality. In the Oolitic limestone of the age of 

 the Lijueiuung group : Burlington, Iowa. 



Cardiomorpha ovata ( n. s.). 



PLATE VII. FIG. 10 a, b, c. 



SHELL ovoid or subcordiform ; valves ovate, slightly ob- 

 lique, a little longer than wide, very gibbous in the middle 

 and towards the umbo ; beak extended and incurved. 



SURFACE marked by numerous fine radiating striae. 



The anterior marginal slope is sometimes nearly straight above and broadly rounded 

 towards the base; differing from pressure, and also without apparent cause. The pos- 

 terior slope is broadly rounded and expanded ; an obtuse subdefined ridge sometimes 

 extending from the beak, on the anterior side, towards the base. 



Fig. 10 a. The right valve of a specimen, which shows an obtuse ridge from the beak 



towards the base. 

 Fig. 10 b. A similar valve, where the surface is more uniformly convex, and without a 



ridge. 



Fig. 10 c. Profile of the same specimen. 



Geological formation and locality. In yellow sandstone of the age of the 

 1 New-York : Burlington, Iowa. 



