728 BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



Length, 12 5 width, 15 ; thickness, 4 millimeters.* 

 Known only from the Independence shales. 



Steophodonta quadrata, n. s. 



Shell small, concavo-convex, quadrate in outline. Cardinal extremi- 

 ties sometimes abruptly produced, sometimes rounded. Ventral valve 

 very convex, flattened, on the umbo, and descending abruptly to the 

 lateral and front margins. Dorsal valve concave, following closely the 

 curvature of the other. Hinge-area common to both valves, wider on 

 ventral, finely striated. Foramen only sufficiently developed to receive 

 the extremities of the bifid cardinal process. Muscular scars faintly 

 impressed, not definitely bounded. 



Surface of ventral valve ornamented by fine radiating striae. From 3 

 to 5 very small strise are implanted between pairs of more prominent, 

 but very slender, filiform, and often slightly interrupted ones. A broad, 

 shallow, mesial sinus sometimes occupies the front half of the valve. On 

 dorsal valve, the strise are subequal, corresponding to the finer ones of 

 the ventral. 



Length, 9; width, 11 5 convexity, 5 millimeters. 



Occurs both at Independence and Eockford. 



Steophodonta aecuata. Hall. 



Stropliodonta arcuata, Hall, Geology of Iowa, 1858, vol. i, part 2, p. 492, plate iii, fig. 

 1 a, 1), c, and 2 a, &. 



Very common in the Eockford shales, and is also found at Independ- 

 ence. 



Steophodonta canace. Hall & Whitfield. 



Stroplwdonta canace, Hall & Whitfield, 23d Ann. Report on State Cab. of New York, 

 p.236,pl.xi, figs. 8-11. 



The specimens in hand present some differences from the Eockford 

 forms. Other sx)ecimens from Independence show more exact agree- 

 ment. 



Steophodonta eeveesa, Hall. 



Stroj}liodonta reversa, Hall, Geology of Iowa, 1858, vol. i, part 2, j). 494, pi. iii, fig. 4 a, d. 



From Independence. Also found at Eockford, where this species is 

 very abundant. 



Oethis infeea, n. s. 



Shell very small, orbicular or subellipticalf valves about equally con- 

 vex. Ventral valve regularly convex, with a slight indication of a 

 mesial fold ; beak prominent, erect or slightly incurved ; hinge-line short, 

 length about equal to a third of the width of shell near the middle; 

 hinge-area narrow. 



*A11 the dimensions given in this paper are taken from average-sized specimens, 

 unless otherwise stated. 



