168 MISSISSIPPIAN BRACHIOPODA 



to the cardinal extremities, the inferior, flatter portion lying nearly at a 

 right angle to the plane of the valve ; the delthyrium large, broadly tri- 

 angular. Internally the cardinal teeth are of moderate size, and are 

 supported by short dental lamella? which are continued anteriorly as low 

 bounding ridges along the lateral margins of the muscular scars ; the 

 muscular scars rather large, sometimes extending anteriorly beyond the 

 middle of the valve, sharply denned all around by a margin which is ab- 

 ruptly raised from the inner surface of the valve, subcordate in outline 

 with a deep and acutely angular emargination in front, divided longitud- 

 inally from the base of the emargination to the beak by a conspicuous, 

 narrowly rounded, median ridge which gradually decreases in size poste- 

 riorly ; each lateral lobe marked by several ill-defined, more or less incon- 

 spicuous, radiating ribs which are strongest anteriorly. 



Brachial valve much more convex than the pedicle, the greatest convex- 

 ity posterior to the middle, the umbo prominent and extending conspicu- 

 ously beyond the cardinal margin, the surface curving abruptly to the 

 margin on all sides but most abruptly to the cardinal margin, somewhat 

 compressed towards the cardinal extremities; the median portion of the 

 valve distinctly flattened posteriorly, the flattened region becoming im- 

 pressed towards the front in a shallow, rounded sinus of moderate width ; 

 the beak pointed and rather strongly incurved ; the cardinal area smaller 

 than that of the opposite valve, its lateral margins sharply defined, con- 

 cave, curving from the hinge-line towards the beak of the opposite valve. 

 The internal characters of the valve not observed. 



Surface of both valves marked by fine, regular, radiating costas which 

 increase by bifurcation and intercalation, about two or three occupying 

 the space of one millimeter at the front of a full grown shell, crossing 

 the costa? are more or less irregular concentric lines of growth which are 

 commonly more conspicuous upon the brachial valve, and are usually 

 somewhat crowded near the margin of full grown shells. The shell sub- 

 stance is pierced by fine, radiating canals or tubules which apparently 

 follow the direction of the radiating costae, along the summit of which 

 they open at intervals. 



Remarks. This is the largest of our species of Schlzophoria, and is a 

 common and characteristic fossil of the Burlington limestone. It may be 

 easily recognized by its large size and the very slight mesial sinus of the 

 pedicle valve. 



Horizon. Burlington limestone. 



