PALAEONTOLOGY OF IOWA. 495 



beneath it, and an indentation on each side in the area mar- 

 gin, with deep pits beneath for the reception of the two 

 horns of the cardinal process of the opposite valve. 



This species resembles in external markings the S. demissa, but the striae are 

 stronger and more angular towards the beaks. The resupinate form and abruptly 

 deflected margins distinguish it from other species. 



This one and the preceeding species show the indented outline of the cardinal area, 

 produced by the impressions of the cardinal process of the opposite valve. 



Fig. 4 a. Dorsal view of a specimen having the margin somewhat irregularly plicated, 

 which is apparently due to accident during the life of the animal. 



Fig. 4 b. Ventral view of the same. 

 Fig. 4 c. Profile view. 



Fig. 4 d. Interior of ventral valve, showing the indented margin of area, bilobed cardi- 

 nal process, the muscular and vascular impressions. 



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

 Hamilton group : Lime creek above Rockford, Iowa. 



PLATE III. FIG. 5 a- fc. 



Strophomena demissa : CONRAD, Journal Academy Natural Sciences, Philadelphia, 1842, 

 Vol. viii, pa. 258, pi. 14, f. 14. 



Strophodonta demissa : HALL, Report of the Regents of the University of New-York, 

 upon the State Collections of Natural History, for 1856, p. 137. 



SHELL seinielliptical, usually wider than high ; hinge line 

 usually equalling or greater than the width of the shell 

 below, the extremities often prolonged into mucronate tips, 

 and sometimes the margins are contracted below, giving a 

 subauriculate character to the shell. Ventral valve very re- 

 gularly convex, sometimes gibbous or ventricose : area well 

 defined, strongly striated vertically, the inner margin cre- 

 nulate. Dorsal valve regularly concave, and following the 

 curvature of the opposite valve : area sublinear, common to 

 both valves ; inner edges strongly crenulate. 



SURFACE marked by strong subangular striae, which are 

 several times dichotomized before reaching the base of the 

 shell, and varying considerably in number and strength in 

 different individuals. Fine concentric strise mark the surface 

 of well-preserved specimens, and, under a lens, the entire 

 surface is punctate. 



