402 MISSISSIPPIAN BRACHIOPODA 



at the base being two-thirds or more of the height; each lateral slope 

 marked by from 18 to 20 simple, depressed, rounded plications which are 

 nearly or quite as strong as those of the opposite valve, they originate 

 along the cardinal margin and grow successively smaller towards the 

 cardinal extremities. Internally the dental lamellae are well developed 

 and rather thick, they extend anteriorly along the floor of the valve for 

 nearly one-half the distance from the beak to the antero-lateral margins, 

 diverging from the beak at an angle much greater than the lateral 

 margins of the mesial sinus ; the muscular scars large and broad, reaching 

 more than half way from the beak to the front of the 'shell, divided 

 along the median line from the beak nearly to the middle of the scar by a 

 low median ridge or septum; the delthyrial plate between the dental 

 lamellae reaches about half way from the apex of the delthyrium to the 

 hinge-line, with the free part of the syrinx extending still further down- 

 ward, it is distinctly keeled longitudinally along the median line, the keel 

 extending out into the free part of the syrinx. 



Brachial valve sometimes nearly or quite as convex as the pedicle, 

 its greatest convexity on the median line at the front of the valve, the 

 surface curving more abruptly to the cardinal margin and becoming 

 more or less compressed towards the cardinal extremities; mesial fold 

 sharply defined to the beak where it is narrow and scarcely elevated above 

 the general surface of the valve, becoming broader and higher anteriorly 

 and rounded on top ; beak short, incurved, the umbonal region projecting 

 somewhat beyond the cardinal margin ; cardinal area narrow, lying nearly 

 in the plane of the valve; each lateral slope marked by 18 or 20 simple 

 plications similar to, but sometimes a little stronger than, those of the 

 opposite valve. 



The minute surface markings are not commonly fully preserved upon 

 the more or less exfoliated specimens of this species, but they are suffi- 

 ciently well preserved in patches to indicate that they are of the typical 

 "twilled cloth" character which is common to the genus. The punctate 

 structure of the shell is also commonly obliterated and has not been 

 observed in any of the specimens studied, but it was doubtless present in 

 the living shell. Concentric lines of growth are commonly present upon 

 the shells, they are irregularly arranged and of varying strength, some of 

 them usually being conspicuous. 



Remarks. This species has been considered as a synonym of S. textus by 

 Schuchert, 1 but the two forms are clearly distinct in several important re- 

 spects. S. subcuspidatus is commonly a narrower shell with proportionally 

 lower cardinal area, which is usually moderately concave instead of flat; 

 a more important difference is in the position of the cardinal area which 

 slopes posteriorly from the hinge-line, making an obtuse angle with the 



l 9th Ann. Rep. N. Y. State Geol., p. 34. (1890.) 



