354 MISSISSIPPIAN BRACHIOPODA 



width 26 mm., thickness 15.5 mm., length of hinge-line 13 mm., height of 

 cardinal area 3 mm. Other specimens are frequently proportionally much 

 broader. 



Pedicle valve with its greatest convexity posterior to the middle, the 

 convexity extending out to the cardinal extremities, the umbonal region 

 prominent but rather narrow, produced posteriorly beyond the hinge- 

 line, the surface sloping abruptly from the umbonal region to the cardinal 

 margin, more gently to the lateral and anterior margins ; beak sharply 

 pointed, narrow and incurved, its tip reaching the plane of the valve ; 

 cardinal area small, arched, the curvature becoming greater towards the 

 beak, the lateral margins defined, sloping steeply from the beak to the 

 cardinal extremities, the delthyrium large, broader than high, its width 

 along the hinge-line a little less than one-half the total length of the 

 hinge-line; lateral slopes each marked by 18 to 20 rounded plications 

 which increase by bifurcation in the umbonal region and less frequently 

 towards the front, they grow successively smaller towards the cardinal 

 extremities where the last ones are nearly obsolete; mesial sinus narrow 

 and well defined towards the beak, where it is without plications, anter- 

 iorly it is ill-defined, shallow and rounded in the bottom, a median 

 plication originates in the umbonal region which increases by bifurcation 

 to two or three at the anterior margin, each lateral slope of the sinus 

 bears about three plications similar in form and size to the middle ones, 

 one or two of which originate from the lateral margins of the bounding 

 plications and the others from the bifurcation of these original ones. 



Brachial valve wider than long, less convex than the pedicle, its 

 greatest convexity near the middle, sometimes a little compressed towards 

 the cardinal extremities; mesial fold defined to the beak but scarcely 

 elevated above the general surface back of the middle of the valve, and 

 only a little elevated in front where it is rounded in contour, marked by 

 plications similar in form and number to those of the sinus in the opposite 

 valve, all of which originate through the division of the single original 

 plication at the beak ; the lateral slopes gently convex, marked by plica- 

 tions similar in form and number to those of the opposite valve. 



The minute surface markings of the shell are not well preserved, but 

 exceedingly fine longitudinal strife are suggested at one point upon the 

 type specimen, and perhaps also some minute concentric lines. A few 

 concentric lines of growth are usually present. 



Remarks. This shell can be easily recognized by its ovoid form, its 

 short hinge-line and its narrow and protuberant umbonal region. In its 

 surface markings it apparently resembles S. grimesi and S. logawi, and its 

 general form is in some respects a miniature of S. grimesi but with pro- 

 portionally shorter hinge-line and finer plications. 



Horizon. Keokuk limestone. 



