398 MISSISSIPPIAN BRACHIOPODA 



tially the same as the lateral boundaries of the sinus ; the transverse del- 

 thyrial plate extends about two-fifths the distance from the apex of the 

 delthyrium to the hinge-line, with the slender, free extension of the syrinx 

 continuing at least one-half as far again, on its outer side the delthyrial 

 plate is flat on either side, with a narrow, squarely elevated median, 

 longitudinal ridge, which continues into the free portion of the syrinx; 

 the muscular scars are narrow and not deeply impressed, and are divided 

 longitudinally by a slight median ridge. 



Brachial valve moderately convex, subsemicircular in outline, with a 

 regularly rounded emargination in front occupying the width of the 

 mesial fold, the surface curving more abruptly to the cardinal margin, 

 compressed towards the cardinal extremities ; mesial fold of moderate 

 height, rounded on top, well defined towards the beak, becoming less 

 well defined anteriorly ; each lateral slope marked by about 20 simple, de- 

 pressed, rounded, radiating plications, similar to those of the opposite 

 valve but more strongly defined. Internally the muscular scars are not 

 strongly impressed, they are divided longitudinally by a low median ridge 

 which extends more than half way from the beak to the anterior margin. 



The minute surface markings and punctate shell structure are not pre- 

 served upon the holotype of this species, this specimen being for the most 

 part an internal cast, the parts of the shell which are preserved being 

 entirely silicified and not preserving their markings. Several concentric 

 lines of growth are shown upon the specimen. 



Remarks. This species is characterized by its proportionally high area 

 and the very oblique anterior slope of the area from the hinge-line. In 

 the small angle between the cardinal area and the plane of the valve, in 

 the shortness of the pedicle valve from beak to front as compared with 

 the height of the cardinal area, and in the slight vertical compression of 

 the pedicle valve towards the cardinal extremities, this species resembles 

 S. extenuatus. It is fundamentally different from 8. extenuatus, however, 

 in its proportionally much higher cardinal area and in the form of the 

 delthyrial plate and syrinx. The species differs from 8. typo, in its flat 

 and more posteriorly sloping cardinal area, and in the form of the del- 

 thyrial plate and syrinx. In some respects this shell approaches more 

 closely to the European 8. cuspidatus than any other of the American 

 species, the flat delthyrial plate with its squarish, median, longitudinal 

 ridge externally, which is produced in the slender free syrinx, being 

 similar in the two species, but the free syrinx is much more elongate in 

 8. cuspidatus; in 8. platypleurus, however, the cardinal area is not convex, 

 the beak is not distorted, and the anterior slope of the area is much 

 greater. 



A specimen from the lower Burlington limestone at Osceola, Missouri, 

 has been referred to this species. It shows only a portion of the in- 



