490 GEOLOGICAL SURVEY OF IOWA. 



The exterior of numerous specimens from a single locality offers little positive 

 difference from the O. iowensis : they are less gibbous, and the sinus in front less 

 deeply marked than in the Iowa specimens; but the form of the muscular and vas- 

 cular impressions in the cast of the dorsal valve is so different, that I have separated 

 these forms as a distinct variety, which may perhaps be found specifically distinct. 



Fig. 5. Cast of the dorsal valve. 



Geological formation and locality. In limestone of the age of the Hamil- 

 ton group : Bakeoven, Illinois. 



Orthis ineqnalis. 



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



SHELL subplano-convex, or depressed hemispherical, semi- 

 elliptical in outline ; hinge line equalling the greatest width 

 of the shell. Dorsal valve very gibbous, greatest convexity 

 near the centre ; umbo prominent ; beak scarcely elevated 

 above the hinge margin. Ventral valve nearly plain, slightly 

 convex towards the beak, flattened at the lateral margins, 

 and slightly concave towards the basal margin, which is not 

 sinuate : area long, narrow ; foramen broad. 



SURFACE marked by alternating larger and smaller striae, 

 and sometimes nearly equal bifurcating striae, which in the 

 casts appear to be fasiculate near the margins : striae curved 

 upwards on the margin of the convex valve, but not run- 

 ning out on the hinge line. 



In the cast, the impressions of striae near the margin present one strong one with 

 two or three finer ones between. 



This species is closely allied to one in the Chemung group of New- York, and may 

 be identical. It differs conspicuously from O. iowensis in its longer hinge line, the 

 nearly flat ventral valve, and absence of sinus in front. 



Geological formation and locality. In fine-grained sandstone of the age 

 of the Chemung group : at Burlington, Iowa. 



Orthis prams. 



SHELL semielliptical, distorted from adhesion by the beak 

 of the ventral valve. Ventral valve convex, gibbous towards 

 the beak ; area high, equal the greatest width of the shell.- 



