S18 MISSISSIPPIAN BRACHIOPODA 



Pedicle valve most convex posterior to. the middle, the surface curving 

 abruptly from the umbonal region to the cardinal margin on each side 

 of the beak, curving more gently to the antero-lateral margins and 

 becoming compressed towards the cardinal extremities; beak small, 

 pointed, moderately incurved; cardinal area of moderate height, arched, 

 the curvature slight in the inferior portion, becoming stronger towards 

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

 the cardinal extremities, the slope becoming more abrupt distally, 

 surface marked by vertical lines, the hinge-margin minutely creriulate, 

 the delthyrium large, broadly triangular; lateral slopes convex towards 

 the middle of the valve, becoming compressed towards the cardinal 

 extremities, each marked by from 16 to 20 depressed rounded plications 

 which grow successively fainter towards the cardinal extremities, in 

 those individuals with excessively elongate hinge-lines becoming nearly 

 or quite obsolete distally, the first one or two plications on each side 

 of the sinus bifurcate close to the beak, the remaining ones are simple ; 

 mesial sinus shallow, rounded in the bottom, defined to the beak, be- 

 coming less sharply defined anteriorly, marked by a median plication 

 which originates near the beak and passes to the anterior margin either 

 as a simple plication or bifurcating near the middle of the valve, on each 

 side are one, or more rarely two, plications which originate from the 

 inner margin of the bounding plications. 



Brachial valve less convex than the pedicle, the greatest convexity near 

 the middle, the surface curving more abruptly to the cardinal margin, 

 compressed towards the cardinal extremities; mesial fold defined from 

 the beak to the front by a pair of furrows which are deeper and stronger 

 than those between the plications, scarcely or not at all elevated above 

 the general surface of the valve through its entire extent, marked by four 

 or five plications similar to those on the lateral slopes which originate 

 from a single one at the beak; lateral slopes marked by plications similar 

 in form and number to those of the opposite valve. 



The minute surface markings consist of exceedingly fine concentric 

 stria? which are sometimes crowded at intervals, and especially towards 

 the anterior margin, to form more or less conspicuous lines of growth. 



Remarks. This species is especially characterized by the great variation 

 it exhibits in the length of the hinge- line, and in the complete, or almost 

 complete, lack of elevation of the mesial fold of the brachial valve above 

 the general surface of the valve. The pedicle valve often more or less 

 closely resembles that of S. osag ens-is, but the plications are finer and more 

 numerous, and the hinge-line is apt to be more elongate, the brachial 

 valves of the two species are less alike. 



The types of this species are all from the white oolitic bed, No. 6 of 

 the Kinderhook section at Burlington, Iowa, but some specimens in the 

 yellow sandstone bed, No. 5, just below, and in the superjacent bed, No. 7, 



