642 GEOLOGICAL SURVEY OF IOWA. 



sinus is well defined near the beak, becoming broad and 

 undefined below, and marked by about seven to twelve 

 dichotomizing plications ; area high, short, with angular 

 margins. Dorsal valve wider than long, with a broad some- 

 what undefined mesial fold, which is scarcely conspicuous 

 above the middle. . 



SURFACE marked by regular rounded plications, eighteen 

 or twenty of which are on each side of the mesial fold and 

 sinus : some of these dichotomize near the beak, and, more 

 rarely, below, while those of the sinus and fold dichotomize 

 near the base. Fine concentric striae give a granulose ap- 

 pearance to the surface, when not worn. 



This species resembles <S. recurvatus of DK KONINCK ( Description des Animaux 

 fossiles, pa. 261, pi. 16, f. 5); but our shell is less gibbous, with a less strongly 

 defined sinus. 



Fig. 2 a. Dorsal valve of this species. 

 Fig. 2 b. Ventral valve. 

 Fig. 2 c. Profile view. 



Geological formation and locality. In the Keokuk limestone : Skunk 

 river, Iowa. 



Spirifer keokuk ( n. s.). 



PLATE XX. Fia. 3 a - d, and 2 d. 



SHELL transversely oval, gibbous : valves nearly equal ; 

 hinge-line equalling or a little less than the width below, 

 obtusely angular or somewhat rounded at the extremities. 

 Dorsal valve convex, very gibbous in the umbonial region : 

 beak prominent and distinctly incurved ; mesial fold more 

 or less elevated below the middle. Ventral valve convex : 

 beak very prominent, gibbous and strongly incurved ; in old 

 shells, closely pressed against that of the other valve : sinus 

 faint and narrow near the beak, deeper and rounded in front, 

 terminating in a triangular projection, fitting into a corre- 

 sponding depression in the front of the opposite valve : area 

 arcuate ; foramen triangular, open. Plications from eighteen 

 to twenty-eight, three or four occupying the sinus and four 



