322 MISSISSIPPIAN BRACHIOPODA 



SPIRIFER LOUISIANENSIS Rowley 

 Plate XL, Figs. 1-23 



1900. Spirifer louisianensis Rowley, Am. Geol., vol. 25, p. 262, pi. 5, figs. 

 18-20, 64. 



Description. Shell small, subtriangular to subsemicircular or subsemi- 

 elliptical in outline, wider than long, the greatest width along the hinge- 

 line, the cardinal extremities more or less acutely angular. The dimen- 

 sions of two examples from the type locality are : length of pedicle valve 

 11.9 mm. and 10 mm., length of brachial valve 10.8 mm. and 8.9 mm., width 

 along hinge-line 13.6 mm. and 13.2 mm., thickness 10 mm. and 8.4 mm., 

 height of cardinal area 2.4 mm. and 2.2 mm., width of mesial sinus in front 

 4 mm. and 3 mm. 



Pedicle valve strongly convex, the greatest convexity posterior to the 

 middle and well back on the umbonal region, the umbo prominent and 

 narrow^, the surface curving abruptly to the cardinal margin on each side 

 of the beak and rather steeply to the antero-lateral margins; beak small, 

 pointed and incurved; cardinal area of moderate height, arched, the 

 curvature becoming greater towards the beak, broadly triangular in out- 

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

 cardinal extremities with a gently convex curve which becomes more ab- 

 rupt distally ; lateral slopes convex antero-posteriorly, the posterior slope 

 to the cardinal margin more abrupt, the slope from the umbo to the car- 

 dinal extremities nearly straight or slightly convex, each lateral slope 

 marked by 10 to 13, usually simple, rounded, radiating plications, the 

 first one or two on each side of the sinus sometimes bifurcating close up 

 to the beak; mesial sinus narrow, shallow, rounded in the bottom, sharply 

 defined by angular plications at the beak, sometimes losing more or less 

 of its definition anteriorly, marked by a median plication which originates 

 near the beak and continues without division to the anterior margin, 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, with a narrow, linear car- 

 dinal area ; mesial fold scarcely elevated above the general surface of the 

 valve except towards the front, bounded by a pair of furrows which are 

 broader and deeper than the other furrows of the valve, marked by from 

 2 to 5 plications, all of which originate from a single one at the beak ; 

 lateral slopes most convex towards the mesial fold, the surface curving 

 more abruptly to the cardinal margin, becoming more or less compressed 

 towards the cardinal extremities, marked by plications similar in form and 

 number to those of the opposite valve. 



Surface of shell, when well preserved, marked by exceedingly fine, reg- 

 ular, concentric strife, about 15 of which occupy the space of 1 mm. and 



