40 MISSISSIPPIAN BRACHIOPODA 



is well preserved, and in the preparation of the above description it has 

 been assumed that the form and contour of the shell, as indicated by the 

 growth lines of this central region, represents the outline of the entire 

 shell. The species has been identified by "Winchell from the Marshall 

 and Waverly formations of Michigan and Ohio, but the specimens so 

 identified are probably different from this Chonopectus sandstone shell. 

 Horizon. Chonopectus sandstone of the Kinderhook. 



ORBICULOIDEA SAMPSONI (Miller) 

 Plate I, Figs. 21, 22 



1891. Discina sampsoni Miller, Adv. Sheets 17th Rep. Geol. Surv. Ind., 



p. 80, pi. 13, figs 10, 12. 



1892. Discina sampsoni Miller, 17th Rep. Geol. Surv. Ind., p. 690, pi. 13, 

 figs. 10, 12. 



Description, Shell small, suboval in outline, the length usually greater 

 than the width. The dimensions of one of the type specimens, a brachiai 

 valve, are : length 12.5 mm., width 12 mm., convexity 2.8 mm. 



Pedicle valve nearly flat, marked by regular, nearly equidistant, con- 

 centric lines, about four of which occupy the space of 1 mm. ; the pedicle 

 opening excentric. 



Brachiai valve conical, with the apex excentric posteriorly and in- 

 clined posteriorly, its distance from the posterior margin being variable, 

 the greatest depth of the valve usually slightly in front of the apex, the 

 surface sloping with a gently convex curvature from the apex to the 

 anterior and antero-lateral margins, becoming slightly concave to the 

 postero-lateral and posterior margins, in some specimens with the beak 

 situated more than usually posterior, the posterior slope is nearly vertical. 

 Surface marked by fine, regular, concentric lines of growth which are 

 somewhat variable in strength, and by exceedingly fine radiating lines 

 which can be detected upon well-preserved specimens only, with a good 

 lens. Internally, a slightly elevated and very narrow ridge extends 

 anteriorly from the apex for about one-third the distance to the anterior 

 margin, and in front of its anterior extremity is a transverse series of four, 

 small, slightly elongated papillae. 



Remarks. The muscular impressions on the interior of the brachiai 

 valve, represented by the slight median ridge and four papillae mentioned 

 above, have been observed in only one example, one of the co-types, 

 where they appear as depressions in the surface of the internal cast 

 where the test has been removed. 



Horizon. Chouteau limestone of the Kinderhook. 



