PALEONTOLOGY OF IOWA. 603 



Spirifer plcnus ( n. s.). 



FLATS XIII. Fia. 4 a - d. 



SHELL large, subglobose, transversely suboval, ventricose : 

 valves nearly equal ; hinge-line scarcely equal to the greatest 

 width of the shell, obtusely angular or a little rounded at 

 the extremities. Dorsal valve very prominent in the middle ; 

 mesial fold obtusely elevated and smoothly rounded. Ventral 

 valve most convex on each side of the sinus below the beak, 

 sloping somewhat abruptly towards the lateral margins : 

 sinus broad, rounded and deep, extending from the beak to 

 the front, where it is prolonged as a broad linguiform ex- 

 tension. On each side of this projection, the margin of the 

 valve is broadly sinuate : beak arched, and projecting beyond 

 the opposite ; area high and distinctly arcuate ; foramen 

 large, triangular, width and height equal, often partially or 

 entirely enclosed by a pseudo-deltidium. 



SURFACE marked by about twenty simple rounded depressed 

 plications on each side of the mesial fold and sinus, crossed 

 by concentric lines of growth. Under a lens, the whole sur- 

 face is seen to be beautifully marked with an extremely fine 

 textile ornament, produced by minute irregularly disposed 

 rounded depressed points and lamellose strise. 



Interior of the ventral valve marked by strong projecting 

 dental lamellse, extending in parallel direction from the base 

 of the foramen nearly to the front of the valve. 



This species may be readily distinguished by its high arcuate area, simple flattened 

 plications, and deep broadly rounded sinus. The interior of the ventral valve also 

 differs from any of the associated species, resembling in this respect S. sowerbyi. 



Fig. 4 a. Ventral view of a specimen of this species. 



Fig. 4 b. Dorsal view and area of same. 



Fig. 4 c. Front view of same. 



Fig. 4 d. Profile view ( the beak of the ventral valve is represented too little arcuate) . 



Geological formation and localities. In the Burlington limestone : Bur- 

 lington, and nine miles below Iowa city, Iowa ; Quincy, Illinois, etc. 



