644 GEOLOGICAL SURVEY OF IOWA. 



The specimens of this shell are usually crushed and disterted; but it may be re- 

 cognized by the rounded extremities, strongly elevated and non-plicate mesial fold, 

 and broad obscure plications. In general appearance it resembles S.pinguis (SowER- 

 BT ), as figured by DE KONINCK (An. foss. pi. 14, f. 4 ); but it is broader in pro- 

 portion to its length, with fewer plications, a more prominent dorsal beak, and the 

 area less distinctly defined. 



Fig. 5. Dorsal view of a specimen distorted by pressure so that the valves are separated, 

 showing the entire area of the ventral valve, the dorsal valve being forced 

 beneath the other along the cardinal line. 



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

 Iowa ; Warsaw, Illinois. 



Spirifer suborbicularis ( n. s.). 



SHELL suborbicular, length and width nearly equal or 

 somewhat wider than long : hinge-line much shorter than 

 the width of the shell; cardinal extremities regularly curved. 

 Dorsal valve convex, gibbous above the middle, with the 

 mesial fold becoming defined below the beak, and somewhat 

 prominent at the base. Ventral valve convex, gibbous above 

 the middle, with elevated umbo and beak abruptly incurved 

 over a narrow area, which in length is about equal to half 

 the width of the shell : foramen with the dental lamellae 

 projecting, and partially closed by a pseudo-deltidium. 



SURFACE marked by broad flattened scarcely defined plica- 

 tions, of which there are seven or eight on each side of the 

 mesial fold and sinus, with two or three more faintly defined 

 on these parts of the shell, and some appearance of a smaller 

 plication in the centre of the sinus. 



This species has usually been referred to S. pinguis ; but it differs in being more 

 orbicular, and having a proportionally more extended hinge-line, as well as the 

 extremely flattened plications, by which it may be distinguished from all the other 

 species which have fallen under my observation from the carboniferous rocks. 



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

 Iowa ; Warsaw, Illinois. 



