PALEONTOLOGY OF IOWA. 641 



Fig. 5 a. View of ventral valve, which is broken transversely across a little below the 

 beak. The ridge extending from the beak is accidental, from pressure on the 

 opposite side. 



Fig. 5 6. Ventral valve of a specimen which is partially concealed at the margins. 



Geological formation and localities. In the Keokuk limestone : Keokuk, 

 Iowa ; Warsaw, Nauvoo, etc. Illinois. 



Spirifer teimimarginata ( n. s.). 



PLATE XX. FIG. 1 a, b, c. 



SHELL semielliptical, length more than three-fourths as 

 great as the width, moderately gibbous in the middle ; ex- 

 tremities rounded ; margins impressed. Dorsal valve with a 

 rounded or subangular mesial fold, little elevated above, and 

 becoming rapidly wider and high towards the front. Ventral 

 valve with a shallow scarcely defined sinus ; beak elevated 

 and incurved : area moderately wide, not extending to the 

 cardinal extremities. 



SURFACE marked by numerous fine dichotomizing plica- 

 tions, which divide once or twice above the middle of the 

 shell, and continue simple, for the most part, to the base ; 

 about twenty on each side, and ten or eleven upon the mesial 

 fold and sinus; concentrically marked by fine undulating 

 striae. 



This species bears some resemblance to 5. inequicostata of OWEN ( Report of Wis- 

 consin, Iowa and Minnesota, pa. 586, pi. v, f. 6); but the plications are more nu- 

 merous than those described of that shell, and are distinctly dichotomizing. 



Fig. 1 a. View of the ventral valve. 



Fig. 1 b. Dorsal view. 



Fig. 1 c. Profile view of the same shell. 



Geological formation and locality. In the Keokuk limestone : Warsaw, 

 Illinois. 



Spirifer rostellatus ( n. s.). 



PLATE XX. Fia.2 a, b, c. 



SHELL longitudinally broad oval, with sides symmetrically 

 curving : hinge-line much shorter than the width of the 

 shell. Ventral valve longer than wide, the beak greatly ele- 

 vated above the opposite valve, incurved : a narrow mesial 

 [ IOWA SURVEY.] 81 



