78 MISSISSIPPIAN BRACHIOPODA 



ened ridges on the interior of the shell, one at each margin of the delthy- 

 rium, near the apex of the beak the dental plates are sometimes slightly- 

 extended anteriorly and join with the median septum to form a shallow, 

 triangular chamber. 



Brachial valve convex throughout except towards the cardinal ex- 

 tremities, which are compressed, the greatest convexity posterior to the 

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

 umbonal region not protuberant beyond the cardinal margin; mesial 

 portion of the valve not differentiated. Internal characters not observed. 



Surface of both valves marked by abruptly elevated, radiating costae 

 which commonly alternate in size, and which increase by bifurcation and 

 implantation. From two to three costae occupy the space of 1 mm., the 

 intercostal spaces usually being a little broader than the costas. The 

 costae are crossed by more or less wrinkle-like, concentric lines of growth 

 which are variously distributed, usually being somewhat more crowded 

 towards the margin of mature shells, and by very fine raised concentric 

 lines which are best developed in the intercostal spaces and which give 

 to the surface, with the costae, a finely reticulate appearance when they 

 are best developed. 



Remarks This species differs from 0. keokuk in its smaller size, its less 

 protuberant umbonal region in the brachial valve, and usually in the more 

 abruptly elevated and more acute beak of the pedicle valve. The relation 

 of the dental plates to the median septum is similar to that in 0. keokuk, 

 these species being somewhat intermediate in condition between the 

 camerate and septate divisions of the genus. Hall and Clarke illustrate 

 an internal view of an example which shows a shallow triangular cham- 

 ber in the beak of the pedicle valve, but other specimens which have 

 been examined indicate that no junction of the dental plates with the 

 septum exists, and consequently no triangular chamber. 



Horizon. Ste. Genevieve limestone and Chester group. 



Family PRODUCTION 

 Genus CHONETES Fischer de Waldheim 



Description. Shell usually small, concavo-convex, subsemicircular in 

 outline, broader than long, the greatest width along the hinge-line. Pedicle 

 valve convex, usually flattened towards the cardinal extremities, with a 

 narrow cardinal area ; the delthyrium closed to the apex by the deltidium ; 

 the lateral margins of the area sharply defined, bearing a single row of 

 hollow, vertical or divergent spines which increase in length towards the 

 cardinal extremities; internally the muscular scars are usually faint, the 

 surface outside the scars is strongly papillose, the papillae appearing in 

 internal casts as pits. Brachial valve concave, with a cardinal area much 

 narrower than that of the opposite valve, and without marginal spines; 



