SPIRIFER 369 



separated by grooves narrower than the plications themselves ; mesial 

 sinus rather shallow or of moderate depth, flattened or rounded in the 

 bottom, rather sharply defined at the beak but becoming ill-defined ante- 

 riorly, marked by 4 or 5 much flattened plications separated by narrow 

 grooves. 



Brachial valve shallower than the pedicle, with a narrow cardinal 

 area ; mesial fold depressed convex, sharply defined but scarcely elevated 

 in the umbonal region, becoming moderately elevated towards the front, 

 marked by much flattened plications similar in form and number to those 

 of the sinus of the opposite valve ; lateral slopes most convex towards the 

 center of the valve, usually becoming a little compressed towards the 

 cardinal extremities, the surface curving abruptly from the umbonal re- 

 gion to the cardinal margin, the slope more gentle to the antero-lateral 

 margins, marked by plications similar in form and number to those of the 

 opposite valve. 



The minute surface markings consist of fine granules which are some- 

 what elongate longitudinally. Rather fine concentric lines of growth are 

 also present which are crowded at intervals, especially towards the front, 

 to form stronger growth lines. 



Remarks. The fine surface markings of this species are usually de- 

 stroyed through the exfoliation of the shell. The species is quite unlike 

 any other Mississippian species of the genus, being especially character- 

 ized by the elevated beak, the high cardinal area, and the much flattened 

 plications of the fold and sinus, these plications being much more de- 

 pressed than those upon the lateral slopes of the shell. 



Horizon . Kinderhook. 



Plate XLII, Figs 17-21 



1906. Spirifer jeffersonensis WelLer, Trans. St. Louis Acad. Sci., vol. 16, 

 p. 444, pi. 6, figs. 18-22. 



Description. Shell below medium size, wider than long, the hinge-line a 

 little shorter than the greatest width, the cardinal extremities rounded. 

 The dimensions of a nearly complete pedicle valve are: length 18 mm., 

 greatest width 22.5 mm., length of hinge-line 21.5 mm., height of cardinal 

 area 7 mm., convexity 11 mm. The dimensions of a brachial valve are: 

 length 15 mm., width 22 mm., convexity 6 mm. 



Pedicle valve strongly convex with the greatest depth towards the um- 

 bonal region, the surface curving gently from the umbo to the anterior 

 and antero-lateral margins and more abruptly to the cardinal margin, 

 scarcely compressed towards the cardinal extremities; the beak obtusely 

 pointed and incurved; cardinal area high, concave, with the curvature 



