320 MISSISSIPPIAN BEACHIOPODA 



riorly, it is subangular or rounded in contour and sometimes becomes 

 quite strongly elevated in front, marked by bifurcating plications to the 

 n amber of from 8 to 12 at the anterior margin, all of which originate 

 from a single one at the beak ; lateral slopes most convex in the umbonal 

 region, the curvature more abrupt to the cardinal margin, compressed 

 towards the cardinal extremities, marked by plications similar in form 

 and number to those of the opposite valve. 



The minute surface markings of the shell are usually more or less com- 

 pletely lacking in the specimens, but a few examples retain fine radiating 

 strias which doubtless originally covered the entire shell surface, and also 

 still finer concentric striae. Concentric lines of growth are more or less 

 strongly marked upon some examples. 



Remarks. This species is characteristic of the Chouteau limestone. It 

 may be easily recognized by the more or less distinct fasciculate arrange- 

 ment of the plications, in which respect it is similar to 8. cameratus of the 

 Pennsylvanian faunas. 8. cameratus, however, attains a much larger size 

 and in it the fasciculate arrangement of the plications is carried to a 

 greater extreme. 



Horizon. Chouteau limestone. 



SPIRIFER BIPLICATUS Hall 

 Plate XXXIX, Figs. 24-26 



1858. Spirifer liplicatus Hall, Geol. Surv. Iowa, vol. 1, pt. 2, p. 519, pi. 



7, fig. 5b (not fig. 5a). 

 1900. Spirifer biplicatus Weller, Trans. St. Louis Acact. Sci., vol. 10, p. 77, 



pi. 2, figs. 6-7. 



Description. Shell small, much wider than long, subtriangular in out- 

 line, with the apex of the triangle truncated at the anterior margin, car^ 

 dinal extremities produced into extremely elongate, mucronate points. 

 The dimensions of the internal cast of a pedicle valve are : length 9 mm., 

 width along the hinge-line 50 mm., convexity 5 mm. The dimensions of a 

 brachial valve are : length 8.5 mm., width along the hinge-line, estimated, 

 42 mm., convexity 4 mm. 



Pedicle valve moderately convex, the greatest convexity posterior to 

 the middle, the umbo rather prominent, the surface of the valve com- 

 pressed laterally; beak small, incurved; cardinal area narrow, lateral 

 margins not sharply defined in the internal cast, the only condition in 

 which the species has been observed, but probably sharply defined in the 

 original condition ; lateral slopes convex antero-posteriorly, the curvature 

 more abrupt to the cardinal margin, compressed towards the cardinal ex- 

 tremities, each marked by about 12 simple, rounded plications which or- 

 iginate along the cardinal margin and grow successively smaller towards 

 the cardinal extremities; mesial sinus narrow and shallow, bounded by a 



