510 GEOLOGICAL SURVEY OF IOWA. 



Spirifer pennatus, 



PLATE V. FIG. 1 a - i. 



Spirifer pennatus and S. ligus : OWEN, Geological Report of Wisconsin, Iowa and 

 Minnesota, pa. 585, pi. iii, f . 3, 4 and 8. 



SHELL variable in form from subglobose to transverse and 

 broadly triangular, often inequilateral ; hinge line extremely 

 extended into wing-like expansions ; valves often nearly 

 equally convex. Ventral valve very gibbous in the middle 

 and on the umbo ; beak much elevated above the hinge line, 

 more or less pointed and slightly incurved ; mesial sinus 

 strongly defined at the margins, widely spreading towards 

 the base, and produced in an angular extension in front : 

 area concave and very large, extending to the extremities 

 of the hinge line, striated vertically and longitudinally ; 

 foramen large, open to the apex, and forming an equilateral 

 triangle. Dorsal valve very gibbous in the middle and upon 

 the umbo, which is abruptly incurved, regularly curved to- 

 wards the baso-lateral margins, and more or less compressed 

 towards the lateral extremities ; mesial fold strongly ele- 

 vated, sometimes a little flattened on the top, and often 

 subangular towards the front and slightly depressed on each 

 side. 



SURFACE marked by about fourteen to twenty-six or more 

 rounded plications on each side of the mesial fold and sinus; 

 those near the centre, to the number of ten or twelve, being 

 much stronger than those upon the extremities, which be- 

 come finally very slender. Plications crossed by closely ar- 

 ranged concentric undulating laminae of growth ; and the 

 entire surface, in perfect specimens, ornamented by slender 

 radiating striae, which become granulose at their junction 

 with the concentric striae. 



This species presents considerable variety of aspect and of surface marking; but 

 after examining a large number of specimens from Rock island, New-Buffalo and 

 Independence, I cannot find means of separation, and am compelled to regard for the 

 present the S. pennatus and <S. ligus of OWEN as only varieties of the same species in 



