304 S. WELLBB FAUNA OF THE FERN GLEN FORMATION 



ent. the ventral margin sloping laterally toward the cardinal extremities, 

 while in 8. marionensis the two margins of the areas are essentially 



parallel. The manner of growth of the shell of this species is account- 

 able for the difference in the form of its cardinal area from that of 

 8. marionensis. The length of the hinge line continues to increase 

 throughout the entire period of growth of the shell, although as the shell 

 approaches maturity the elongation is proportionally less rapid; at the 

 same time the height of the area is continuously increasing, so that the 

 mature shell possesses the broadly subtriangular cardinal area. Another 

 feature of this species which can sometimes be detected is the presence 

 of slight crenulations upon the hinge line. The larger individuals ap- 

 proach 8. imbrex, of the Burlington limestone, but they are more coarsely 

 ribbed and exhibit less bifurcation of the plications. 



SPIRIFER GRIMESI Hall 

 Plate 13. figures 1-2 



1S5S. Spirifer grimesi Hall. Geological Survey of Iowa, volume 1. part 2. page 



604. plate 14L, figures 1-5. 

 1895. spirifer grimesi H. & C. Paleontology of New York, volume 8, part 2 



plate 30, figures 8, 16-19. 



Description. — Shell large, varying from longitudinally to transversely 

 subelliptical in outline, the length greater or less than the width, hinge 

 line shorter than the greatest width, cardinal extremities obtusely angu- 

 lar or somewhat rounded. Pedicle valve strongly convex: beak rather 

 large, incurved: cardinal area of moderate width, nearly flat toward the 

 hinge line, becoming more strongly concave toward the beak, its ventral 

 margin sharply denned, sloping regularly on each side from the beak 

 toward the cardinal extremities, the slope becoming much more abrupt 

 as it approaches the extremities: mesial sinus rather broad and shal] 

 rounded or more or less angular in the bottom, originating at the beak, 

 where it is quite sharply defined, losing its definition anteriorly; lateral 

 slopes convex, becoming more or less flattened toward the cardinal ex- 

 tremities. Brachial valve about as strongly convex as the pedicle; the 

 mesial fold broad, ill defined, becoming more strongly elevated and often 

 somewhat angular toward the anterior margin: lateral slopes convex, 

 sometimes becoming more or less flattened toward the cardinal extremities 

 Surface of each valve marked by eighty or more depressed rounded bifur- 

 cating plications, about twenty or twenty-five of which occupy the fold 

 and sinus. The minute surface markings consist of very fine radiating 

 striae, about six or eight of which occupy each plication, and by still finer 

 concentric striae, giving to the surface of perfectly preserved shells a 



