380 MISSISSIPPIAN BRACHIOPODA 



substance present, it is possible that one or two additional plications 

 might be present towards the cardinal extremities of each lateral slope 

 of the valves. The state of preservation is also responsible for all absence 

 of the minute surface characters of the shell. In its general aspect the 

 species is similar to S. suborbicularis except in its much smaller size and 

 in the distinct median plication of the fold. 

 Horizon. St. Louis limestone. 



BRACHYTHYRIS SEMIPLICATA (Hall) 

 Plate LIX, Figs. 15-19 



1860. Spirifer semiplicata Hall, 13th Rep. N. Y., State Cab. Nat. Hist., 



p. 111. 

 1866. Spirifer cooperensis, Meek and Worthen, Geol. Surv. 111., vol. 2, p. 



155, pi. 14, figs. 5a-b. 



Description. Shell small, wider than long, the hinge-line a little 

 shorter than the greatest width, the cardinal extremities rounded. The 

 dimensions of a nearly complete specimen are : length of pedicle valve 8.5 

 mm., length of brachial valve 7 mm., greatest width 9.1 mm., length of 

 hinge-line 6 mm., thickness 6.4 mm. 



Pedicle valve subpyramidal in form, the greatest depth posterior to the 

 middle in the umbonal region, the surface curving abruptly to the car- 

 dinal margin on each side of the beak and more gently to the lateral and 

 anterior margins, not compressed towards the cardinal extremities ; beak 

 blunt and short, incurved; cardinal area of moderate size, concave, its 

 lateral margins not sharply defined, its surface curving into the lateral 

 slopes of the valve with but slight interruption ; mesial sinus originating 

 as a flattening of the valve in the umbonal region, becoming broad ante- 

 riorly but remaining very shallow and flat, marked by a single low, rather 

 faint, median plication, which originates near the middle of the valve; 

 the lateral slopes rather sharply differentiated from the mesial sinus, each 

 marked by about four simple, obscure, rounded plications which become 

 obsolete a little past the middle of the valve. 



Brachial valve depressed convex, much shallower than the pedicle, its 

 greatest depth posterior to the middle, the surface curving rather ab- 

 ruptly to the cardinal margin and more gently to the lateral and anterior 

 margins, a little compressed towards the cardinal extremities; the beak 

 obtuse, only slightly produced beyond the cardinal margin; the mesial 

 fold obscure in the posterior half of the valve, broad, flat and only slightly 

 elevated in front, marked by an obscure mesial furrow ; the lateral slopes 

 marked by obscure plications entirely similar to those of the opposite 

 valve. 



The finer surface markings consist of fine, concentric lines of growth 

 upon each valve. 



