312 MISSISSIPPIAN BRACHIOPODA 



SPIRIFER PIKENSIS Rowley 



Plate XXXVIII, Figs. 1-5 



1902. Spirifer pikensis Rowley, Am. Geol., vol. 29, p. 309, pi. 18, figs. 17-22. 



Description. Shell large, broader than long, the hinge-line a little 

 shorter than the greatest width, the cardinal extremities obtusely angular. 

 The dimensions of a nearly perfect specimen, the holotype, are: length 

 of pedicle valve 62 mm., length of brachial valve 53.4 mm., greatest 

 width 80.5 mm., thickness 36 mm., length of hinge-line 70 mm., height of 

 cardinal area 5 mm. 



Pedicle valve moderately convex, the surface curving abruptly from 

 the umbonal region to the cardinal margin on each side of the beak, and 

 sloping w r ith a gently convex curve to the lateral and antero-lateral mar- 

 gins, only slightly compressed towards the cardinal extremities; mesial 

 sinus originating at the beak where it is narrow, angular and rather 

 sharply defined, less well defined anteriorly where it is narrowly rounded 

 in the bottom and rapidly increases in depth, becoming profound towards 

 the anterior margin, greatly produced in front in a conspicuous lingual 

 extension which is narrowly rounded at its extremity; the beak rather 

 small and pointed, moderately incurved at the apex ; cardinal area rather 

 low, gently concave except just under the beak, where the curvature 

 becomes much stronger, the lateral margins sharply defined and sloping 

 with a gentle curvature from the beak to the cardinal extremities, the 

 delthyrium rather small, broader than high. Surface of the valve marked 

 by rounded, radiating plications which occupy the lateral slopes and the 

 mesial sinus, the largest plications occur upon the lateral slopes adjacent 

 to the sinus where they sometimes measure 3 mm. from center to center 

 at the front margin, towards the cardinal extremities they gradually 

 decrease in size, about 25 being present upon each lateral slope, about 16 

 occupy the sinus, all the larger plications bifurcate once or sometimes 

 twice near the beak, with an occasional bifurcation further towards the 

 front, sometimes in front of the middle of the valve. 



Brachial valve more convex than the pedicle, its greatest depth at the 

 front of the mesial fold, surface of the valve sloping posteriorly from 

 the point of greatest convexity along the median line in nearly a straight 

 line for about two-thirds the length of the valve, from which point it is 

 convexly curved to the beak ; from the median line the surface curves 

 abruptly on either side anteriorly, to the lateral borders of the mesial 

 fold, beyond which lines the surface is gently convex, becoming some- 

 what compressed towards the cardinal extremities, curving more ab- 

 ruptly to the cardinal margin on either side of the beak; mesial fold 

 only slightly elevated near the beak, but rapidly becoming highly elevated 

 in the anterior half of the valve where it is narrowly rounded across 

 the top; beak small and rather strongly incurved, the umbonal region 



