342 MISSISSIPPIAN BRACHIOPODA 



SPIRIFER BRECKENRIDGENSIS n. sp. 

 Plate XLIV, Figs. 20-24 



Description. Shell below medium size, length and width sub-equal, or 

 the length greater than the width, the hinge-line shorter than the greatest 

 width, the cardinal extremities obtusely angular, the greatest width near 

 the mid-length of the shell. The dimensions of two nearly complete in- 

 dividuals are : length of pedicle valve 20 mm. and 18 mm., length of 

 brachial valve 15.5 mm. and 14.4 mm., greatest width 17.8 mm. and 18 

 mm., thickness 14 mm. and 13 mm., length of hinge-line 14.2 mm. and 

 13.4 mm. 



Pedicle valve rather strongly convex, the greatest convexity posterior 

 to the middle, the surface curving abruptly from the umbonal region to 

 the cardinal margin with little or no compression towards the cardinal 

 extremities, and more gently to the lateral margins, from the beak to the 

 front margin the surface curves with regularly decreasing convexity; 

 mesial sinus originating at the beak where it is very small but sharply 

 defined, becoming rounder and somewhat less sharply defined anteriorly 

 where it is of moderate depth, produced anteriorly in a rather short, 

 rounded, lingual extension; the beak pointed and strongly incurved; the 

 cardinal area of moderate height, concave, with the curvature increasing 

 towards the beak, the flatter inferior portion sloping posteriorly at an 

 angle of about 135 degrees to the plane of the valve, the lateral margins 

 sharply defined, sloping from the beak to the cardinal extremities with a 

 slightly sigmoidal curvature ; the delthyrium rather large, its base occu- 

 pying about one-fourth the length of the hinge-line. Surface of the 

 valve marked by rounded plications, the largest of which are usually 

 those bounding the mesial sinus; in the umbonal region of the mesial 

 sinus a median plication originates as a faint rib which grows regularly 

 larger anteriorly until it sometimes attains nearly the size of those 

 bounding the sinus, upon each lateral surface of the sinus is a single 

 somewhat smaller plication which originates in the umbonal region from 

 the inner margin of the bounding plication; each lateral slope of the 

 valve is marked by nine or ten simple plications which originate at or 

 near the beak, these plications grow regularly smaller towards the cardinal 

 extremities, the last one or two being very faint ; the surface also marked 

 by sublamellose, concentric lines of growth which become stronger and 

 more crowded towards the front margin. 



Brachial valve a little less convex than the pedicle, its greatest con- 

 vexity near or a little posterior to the middle, the umbo a little prominent 

 and produced slightly posteriorly beyond the hinge-line, the surface a 

 little compressed towards the cardinal extremities, aside from which it 

 is rather regularly convex; mesial fold originating at the beak as a 

 median plication scarcely elevated above those on either side, becoming 



