PALAEONTOLOGY OF IOWA. 713 



spines, which are sometimes numerous : upper half of the 

 shell covered by strong concentric wrinkles. Surface of 

 dorsal valve marked by strong rounded costse and equally 

 strong concentric wrinkles, giving, as in the opposite valve, 

 a subnodose appearance. Entire surface of both valves, in 

 well-preserved specimens, marked by fine concentric striae. 



This shell is doubtless the same as that identified by M. DE KONINCK with the 

 P. costatus of Europe; but if the figures of SOWERBT and DE VERXEUIL are to be 

 relied upon as characteristic, our shell should be regarded as distinct. In deference 

 to the authority of M. DE KONINCK, however, I have referred the specimens under 

 examination to that species, merely indicating them as a well-marked variety, which 

 I believe future examinations will prove a distinct species. 



Fig. 3 a. Ventral valve of a specimen preserving a greater number of spine-bases than 

 usual. 



Fig. 3 b. Profile view of the same. 



Fig. 4 a. Ventral valve of a strongly costate specimen with few spines, and a well-marked 

 mesial sinus. 



Fig. 4 b. Dorsal view of the same [ the radiating costse and concentric wrinkles are not 

 properly shown in this figure] . 



Fig. 4 c. Profile view, showing the curving row of spines which extend from near the 

 beak. 



Geological formation and localities. In the Coal measures of Ohio, Illi- 

 nois, Missouri and Iowa. 



Orthis robusta ( n. s.). 



PLATE XXVIII. FIG> 5 a, b, c, d. 



SHELL resupinate, semielliptical in outline, plano-convex. 

 Dorsal valve extremely convex, gibbous in the middle and 

 towards the umbo, broadly and evenly curving towards the 

 front. Ventral valve nearly flat at the sides, slightly concave 

 in the middle, with the beak and umbo elevated and some- 

 what distorted ; beak not incurved : area moderately high, 

 plane, extending to the cardinal extremities, which are as 

 wide as the greatest width of the shell below : foramen 

 narrow, closed nearly to the base, where there is a narrow 

 transverse semioval space which is nearly filled by a cardi- 

 nal process from the opposite valve, having a central groove 

 which is partially visible when the valves are a little se- 

 parated. 



[ IOWA SURVEY.] 90 



