PALEONTOLOGY OF IOWA. 507 



Spirifer bimesialis (n.s.). 



PLATE IY. FIG. 6 b, c, d. 



SHELL extremely transverse ; width more than twice the 

 length of the shell; hinge line extremely extended and 

 produced into mucronate points ; front emarginate. Ventral 

 valve gibbous in the middle, curving to the front, and slop- 

 ing gently towards the lateral extremities, where it is nearly 

 flat ; beak much elevated above the opposite valve and very 

 slightly incurved ; area concave, moderately high, and much 

 extended laterally ; sinus strongly defined, subangular, with 

 a simple plication in the bottom. Dorsal valve less elevated 

 than the opposite, convex in the middle, curving towards 

 the beak ; mesial fold divided by a slight depression along 

 the centre, and emarginate in front for the reception of the 

 mesial extension of the opposite valve. 



SURFACE marked by about twelve or thirteen, or more, 

 simple plications on each side of the mesial fold and sinus : 

 plications abruptly rounded on the upper side, crossed by 

 closely arranged imbricating lamellose striae which are ab- 

 ruptly arched on the plications and in the mesial sinus, 

 while they are doubly arched upon the mesial fold. 



This species bears considerable resemblance to 5. submucronatus, fig. 3 ; but the 

 specimens are less gibbous, and the cardinal extremities less abruptly but more ex- 

 tremely extended. It bears some resemblance to S.muralis (Ds VERNEUIL, Russia 

 and the Ural mountains, pa. 171, pi. v, f. 5); but the hinge extremities are less ab- 

 ruptly mucronate, the area less high, and the mesial fold double or depressed in the 

 middle. 



Fig. 6 b. Dorsal valve of an apparently full-grown specimen. 



Fig. 6 c.- Cardinal view, showing the area, foramen, and part of the dorsal valve. 



Fig. 6 d. Ventral valve of another specimen, showing the small plication in the centre 

 of the mesial sinus. 



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

 Hamilton group : Independence, Iowa. 



