o28 MISSISSIPPIAN BRACHIOPODA 



the valve marked by about 75 or 80 flattened, radiating plications at the 

 margin, those in the central portion of the valve, in the sinus and for 

 some distance on either side, increase by bifurcation near the beak or 

 more rarely between the beak and the middle of the valve, from 16 to 20 

 plications occupying the sinus in front with about 30 upon each lateral 

 slope ; rather fine, crowded, subimbricating concentric markings cross the 

 radiating plications, usually being more conspicuous towards the front 

 margin, in addition to which are occasional much stronger concentric 

 lines of growth. 



Brachial valve less convex than the pedicle, its greatest convexity near 

 the middle, the surface convex antero-posteriorly, but gently concave 

 from the median region in the long slope to the cardinal extremities, the 

 curvature to the cardinal margin shorter and more abrupt than that to 

 the anterior margin ; mesial fold originating at the beak where it is rather 

 sharply defined by deeper furrows, but scarcely elevated above the gen- 

 eral surface of the valve, anteriorly it is rounded, becoming ill-defined, 

 and is essentially undifferentiated from the lateral slopes of the valve; 

 the beak incurved, the umbonal region but little protuberant beyond the 

 cardinal margin; the cardinal area narrow, with sharply defined margin. 

 Surface marked by radiating plications and concentric lines entirely sim- 

 ilar to those of the opposite valve. 



Remarks. This species seems to be a rather rare form and has some- 

 times been confused with S. tenuico status. It differs from that species 

 in having rather coarser and much less angular plications, and less con- 

 spicuous, sublamellose, concentric markings. The number of plications 

 in the specimen mentioned above is fully equal to the number usually 

 recognized in examples of S. tenuicostatus, but it a broader specimen 

 than is often found in the other species and consequently the plications 

 themselves are somewhat wider, about 8 or 9 in the space of 10 mm., in the 

 median portion of the front margin, while S. tenuicostatus usually has about 

 10 to 12 in the same space. The species has much the form and proportions 

 of S. forbesi, but that species is commonly somewhat more extended along 

 the hinge-line and has coarser and less commonly bifurcating plications. 

 In 8. tenuicostatus the plications bifurcate more frequently towards the 

 front. 



Horizon. Keokuk limestone and Warsaw formation. 



SPIRIFER TENUICOSTATUS Hall 

 Plate XLII, Figs. 4-16 



1858. Spirifer tenuicostatus Hall, Geol. Iowa, vol. 1, pt. 2, p. 662, pi. 23. 



figs. 8a-c. 

 1906. Spirifer siibcequalis Beede, 30th Ann. Hep. Geol. Surv. Ind., p. 1316, 



pi. 20, figs. 4-4b. 



