648 GEOLOGICAL SURVEY OF IOWA. 



broadly curving to the base : mesial fold very prominent, 

 extremely elevated and subangular in front, not defined at 

 the margins. Ventral valve very gibbous at the sides, marked 

 by a broad deep undefined mesial sinus, which, in the 

 middle of the shell, occupies fully one-third of the width, 

 sloping abruptly to the cardinal extremities, and extremely 

 produced and elevated in front in a subtriangular extension : 

 umbo moderately gibbous : beak neatly defined and incurved 

 over the foramen : area moderately wide, concave, the sides 

 nearly parallel, sharply defined on the outer margin, verti- 

 cally and longitudinally striate ; foramen forming a nearly 

 equilateral triangle, the base being a little longer than the 

 sides. 



SURFACE of shell, including the mesial fold and sinus, co- 

 vered by small rounded bifurcating plications, some of which, 

 on the sides of the mesial sinus, are finer than the others : 

 minute structure undetermined, but apparently striato- 

 punctate. 



This species is, in the Keokuk limestone, the representative of the S.grimesi of 

 the Burlington limestone. It differs from that in being usually more extended on the 

 hinge-line and less produced in front, while the mesial sinus and elevation are more 

 distinctly defined; the plications are likewise somewhat stronger, and of different 

 character. The area of the muscular and vascular impressions of the ventral valve 

 are proportionally shorter and broader. 



This is one of several species which have usually been referred to Spirifer striatus 

 of MARTIN; and it is without doubt a closely allied species, but differs in the pro- 

 minence of the baso-lateral margins, in the depth of the mesial sinus and prominence 

 of the mesial fold, while the foramen appears to be proportionally larger, and the 

 plications stronger and fewer than in shells of Spirifer striatus of the same size. 



PLATE xxi, fig. 1 a. Dorsal view of a large individual, natural size. 

 PLATE xxi, fig. 1 b. Ventral view of the same specimen. 

 PLATE xxi, fig. 2. Cardinal view of a separate ventral valve. 

 PLATE xx, fig. 7. Profile view of the specimen fig. 1 a, b, of pi. xxi. 



Geological formation and locality. In the Keokuk limestone ; near 

 Nauvoo, Illinois. 



