350 MISSISSIPPIAN BRACHIOPODA 



SPIRIFER KEOKUK Hall 

 Plate XLIX, Figs. 1-14 



1852. Spirifer attenuatus Owen, Geol. Rep. Wis., Iowa, and Minn., tab. 



3A, fig. 8. 

 1858. Spirifer keokuk Hall, Geol. Iowa, vol. 1, pt. 2, p. 642, pi. 20, figs. 



3a-d, 2d. 

 1883. Spirifera Keokuk Hall, Rep. N. Y. State Geol. for 1882, pi (30) 55, 



figs. 21-22 (not figs. 23-24). 

 1894. Spirifer Keokuk Hall and Clarke, Int. Study of Brach., pt. 2, pi. 



27, figs. 14-15. 



1894. Spirifera keokuk Keyes, Mo. Geol. Surv., vol. 5, p. 81, pi. 40, fig. 2. 



1895. Spirifer Keokuk Hall and Clarke, Pal. N. Y., vol. 8, pt. 2, pi. 30, 



figs. 21-22 (not figs. 23-24). 



Description. Shell of medium size, wider than long, greatest width at 

 or near the hinge-line, the cardinal extremities more or less nearly 

 rectangular, sometimes rounded in the narrower specimens. The dimen- 

 sions of two examples are : length 26.5 mm. and 26.8 mm., width 31 mm. 

 and 35 mm., thickness 19.5 mm. and 19.5 mm. 



Pedicle valve strongly convex, the greatest convexity posterior to the 

 middle, the umbonal region broadly convex, the surface more or less 

 compressed towards the cardinal extremities; beak pointed and rather 

 strongly incurved ; cardinal area concave, becoming more strongly curved 

 as it approaches the beak, the inferior portion sloping posteriorly at an 

 angle of about 135 degrees to the plane of the valve, about 4 to 5 mm. 

 in height, the lateral margins sharply defined, sloping from the beak 

 to the cardinal extremities first with a convex and then with a concave 

 curvature ; lateral slopes of the valve convex on either side of the sinus, 

 curving abruptly to the cardinal margin, the curvature becoming concave 

 as it approaches the cardinal extremities, each marked by from 11 to 15 

 rounded plications which become progressively smaller towards the car- 

 dinal extremities, all of them are simple except occasionally those which 

 form the lateral boundaries of the sinus ; mesial sinus shallow and rounded 

 in the bottom near the beak, becoming deeper and broader anteriorly, 

 sometimes produced anteriorly into a conspicuous nasute extension; near 

 the beak a single median plication originates which usually continues to 

 the anterior margin as a simple rounded rib, increasing regularly in size, 

 occasionally this median rib bifurcates and continues to the anterior 

 margin as two smaller rounded plications ; on each side of the median rib 

 may be seen one or perhaps two, usually weaker, plications upon the lateral 

 slopes of the sinus, which originate from the inner lateral margins of the 

 plications bounding the sinus. 



Brachial valve a little less convex than the pedicle, greatest convexity 

 near the middle, the surface sloping more abruptly from the umbo to the 

 cardinal margin, becoming somewhat compressed towards the cardinal 



