370 MISSISSIPPIAN BRACHIOPODA 



increasing towards the beak, the lateral margins not sharply defined, the 

 surface of the area rounding into the surface of the lateral slopes ; the 

 delthyrium large and broadly triangular ; the mesial sinus of moderate 

 depth, rounded in the bottom, well defined laterally and continuing to 

 the beak, it is marked by two more or less indistinct, sometimes nearly 

 obsolete plications; lateral slopes each marked by from 8 to 12, usually 

 9 or 10 simple, rounded plications. 



Brachial valve subelliptical in outline, moderately convex, the greatest 

 depth posterior to the middle, the surface curving abruptly to the car- 

 dinal margin and more gently to the anterior and antero-lateral margins, 

 very slightly or not at all compressed towards the cardinal extremities ; 

 mesial fold sharply defined to the beak but scarcely elevated above the 

 general surface in the posterior half of the valve, becoming moderately 

 elevated anteriorly, it is marked by three or four, low, rounded, indis- 

 tinct plications which often appear to be almost obsolete ; the lateral 

 slopes marked by simple, rounded plications entirely similar in form and 

 number to those of the opposite valve. 



The finer surface markings consist of minute concentric lines of 

 growth, which are usually nearly or quite destroyed by the exfoliation of 

 the shell. 



Remarks. In its high cardinal area and inconspicuous plications on 

 the fold and sinus this species suggests the genus Syringothyris, but it lacks 

 the syrinx and the punctate shell structure of that genus. At one time 

 it was believed that certain specimens in the Bushberg sandstone, in 

 which the presence of a syrinx is clearly shown, were examples of this 

 species, and that the species should therefore be transferred to Syringothy- 

 ris, 1 but later observations have demonstrated that the two forms are not 

 the same, and that this species is a true Spirifer and the other a genuine 

 Syringothyris. 



Horizon. Glen Park limestone of the Kinderhook. 



Genus BRACHYTHYRIS McCoy 



Description. Shells varying in size from small to large, the length and 

 width subequalj the outline ovate to subcircular; the hinge-line straight, 

 much shorter than the greatest width of the shell, the cardinal extremi- 

 ties rounded. Mesial sinus of the pedicle valve and the fold of the 

 brachial valve more or less well developed. Surface of both valves 

 marked by radiating plications which are simple upon the lateral slopes 

 of the valves, those of the fold and sinus increasing in number in passing 

 from the beak to the front margin. The pedicle valve with the beak 

 usually only moderately elevated above the hinge-line, and the cardinal 

 area rather strongly arched. Internally, the dental lamellae supporting 



l Jour. Geol., vol. 17, p. 272. (1909.) 



