662 GEOLOGICAL SURVEY OF IOWA. 



Spirifer tenuicostatus ( n. s.). 



PLATE XXIII. FIG. 8 a, b, c. 



SHELL semielliptical, nearly twice as wide as long : valves 

 nearly equally convex. Dorsal valve more or less gibbous in 

 the middle ; the mesial elevation not defined, and scarcely 

 distinct from the general convexity of the shell above the 

 centre, becoming prominent on the lower half of the shell : 

 cardinal margin marked by a narrow linear area. Ventral 

 valve with the beak moderately elevated above the opposite 

 and slightly incurved : mesial sinus often well defined at 

 its margins on the upper half of the shell, and becoming 

 broader and undefined on the lower half : area of moderate 

 height, concave ; foramen large. 



SURFACE, including the mesial sinus and elevation, marked 

 by fine equal striee, some of which bifurcate on the upper 

 half of the shell ; those occupying the mesial fold and sinus 

 being from ten to fourteen, resulting from the dichotomizing 

 of two or three at the apex : about twenty-four to twenty- 

 eight plications on either side of the mesial fold and sinus. 

 Plications crossed by fine elevated closely arranged concen- 

 tric strise, with stronger distant imbricating laminaa of 

 growth. 



This species varies in its degree of gibbosity, but is usually distinguished by its 

 moderate convexity and the fine equal plications, which are scarcely interrupted by 

 the bifurcations, and the closely arranged concentric striae by which it differs from 

 any other species observed. It appears to be common to the Warsaw and Keokuk 

 limestones, unless there be some error in the marking of specimens. Some specimens 

 from Keokuk are undistinguishable from the Warsaw specimens, and it is not im- 

 probable that all are from beds of the age of the Keokuk limestone. 



Fig. 8 a. Dorsal view, showing the beak and part of the area and foramen of the 



opposite valve. 

 Fig. 8 b. Ventral valve, in which the lines of growth and concentric striae are well 



preserved. 



Fig. 8 c. Profile view of the same. 



Geological formation and localities. In the Keokuk and Warsaw lime- 

 stones : Keokuk, Iowa ; Warsaw, and Dallas, Illinois. 



