SPIRIPER 351 



extremities ; mesial fold scarcely elevated above the general surface near 

 the beak, often becoming rather highly elevated anteriorly, always rounded, 

 marked by a shallow rounded median furrow corresponding with the 

 median plication of the sinus, and by two or sometimes three plications 

 on each side; lateral slopes marked by rounded plications which are 

 similar in form and number with those of the opposite valve. 



Surface of both valves marked by fine concentric lines which are more 

 crowded and conspicuous towards the anterior margin and by occasional 

 stronger lines of growth, also by very minute longitudinal striae which 

 give to the perfectly preserved surface, when viewed under a lens, a 

 finely reticulate appearance. 



Remarks. Shells from various horizons in the Mississippian have com- 

 monly been referred to 8. keoltuk, but many of them have been incorrectly 

 identified. The species seems to be closely confined to the Keokuk lime- 

 stone and often occurs in great numbers in one zone near the summit of 

 the limestone beds of the formation. The species may be easily distin- 

 guished from other related forms by reason of the rotund convexity of 

 the umbonal region of the pedicle valve, and by the less angular sinus 

 of the same valve near the beak. 



Horizon. Keokuk limestone. 



SPIRIFER PLOYDENSIS n. sp. 

 Plate XLIX, Figs. 15-19 



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

 at or near the hinge-line, the cardinal extremities nearly rectangular. 

 The dimensions of two individuals are : length of pedicle valve 35.5 mm. 

 and 31.5 mm., length of brachial valve 31 mm. and 25 mm., width 43 mm. 

 and 35 mm., thickness 24 mm. and 22 mm., height of cardinal area 4 mm. 

 and 3.5 mm. 



Pedicle valve strongly convex in the umbonal region, the greatest con- 

 vexity posterior to the middle, the surface curving abruptly to the car- 

 dinal margin, more gently to the antero-lateral margins, somewhat com- 

 pressed towards the cardinal extremities ; beak small, attenuate, incurved ; 

 cardinal area of moderate height, arched, the curvature increasing to- 

 wards the beak, the lateral margins sharply defined, sloping from the 

 beak to the cardinal extremities in nearly a straight line or with a gently 

 concave curve ; lateral slopes convex towards the median sinus, becoming 

 slightly compressed towards the cardinal extremities, each marked by 

 from 15 to 20 depressed rounded plications, one or more of which may 

 bifurcate between the umbonal region and the anterior margin, they grow 

 successively smaller towards the cardinal extremities, the last 3 or 4 

 being very faint or sometimes almost obsolete; mesial sinus originating 

 at the beak where it is simple and sharply defined, anteriorly it becomes 



