376 MISSISSIPPIAN BRACHIOPODA 



Remarks. This species occurs somewhat commonly in the Burlington 

 and Keokuk limestones, and has been identified by Beede from the Salem 

 limestone of Indiana. 1 This last identification, however, is incorrect 

 since the figure given to represent the species shows a shell with strong 

 and elongate dental lamelke. In general form the species somewhat 

 resembles 8. subcardiiformis and S. gurleyi; from the first of these it differs 

 in its much longer cardinal area, and from both it differs in having much 

 less strongly developed plications upon the lateral slopes, especially of 

 the pedicle valve, and in the almost obsolete plications of the fold and 

 sinus. 



Horizon. Burlington and Keokuk limestones. 



BRACHYTHYBIS SUBCARDIIFORMIS (Hall) 

 Plate LX, Figs. 1-15 



1858. Hpirifer subcardiiformis Hall, Geol. Iowa, vol. 1, pt. 2, p. 660, pi. 23, 



figs. 6a-b. 

 1880. Spirifer subcardiiformis "White, Cont. to Inv. Pal., No. 8, p. 165, 



pi. 41, figs. 2a-c. 

 1906. Spirifer subcardiiformis Beede, 30th Ann. Rep. Geol. Surv. Ind., 



p. 1313, pi. 21, figs. 2-2b. 



Description. Shell of medium size or larger, suborbicular in outline, the 

 greatest width near the mid-length of the shell, the true hinge-line short, 

 the cardinal extremities rounded. The dimensions of two individuals are : 

 length 43 mm. and 28 mm., width 43.5 mm. and 32 mm., width of true 

 cardinal area along the hinge-line 15.5 mm. and 10 mm., thickness 27.3 

 mm. and 17 mm. 



Pedicle valve with its greatest convexity posterior to the middle, the 

 surface curving abruptly from the umbonal region to the cardinal mar- 

 gin and more gently to the anterior and lateral margins, the convexity of 

 the valve extending to the cardinal extremities; beak pointed and in- 

 curved ; cardinal area small, concave, becoming more curved towards the 

 beak, sloping posteriorly at an angle of about 115 degrees to the plane of the 

 valve, its width along the hinge-line about one-third the total width of 

 the shell, the central half of its width occupied by the large triangular 

 delthyrium so that the actual surface of the area is limited to two narrow 

 bands which pass obliquely from the hinge-line on each side of the 

 delthyrium to the beak, the lateral margins of the area are well defined 

 and beyond them the surface of the valve is abruptly depressed into a 

 concave, false cardinal area broader than the true area, which rounds 



1 Spirifer suborMcularis Beede, 30th Ann. Rep. Geol. Surv. Ind., p. 1312, pi. 19 

 figs. 3-3a; pi. 21, fig. 4. (1906.) 



