224 MISSISSIPPIAK BRACHIOPODA 



The later authors who have referred to the species have placed it in the 

 genus Pugnax, but the distinct median septum of the brachial valve, which 

 is divided above for the support of the hinge-plate is entirely different from 

 those characters in Pugnax. It differs from Camarotozchia in the arching 

 over of the cavity between the divisions of the median septum by the un- 

 divided basal portion of the hinge-plate, and by the consequent absence of 

 the open crural cavity at the base of the hinge-plate. These internal char- 

 acters, as well as the external aspect of the shell, distinguish the species 

 from any of the recognized generic divisions of rhynchonelloid shells and 

 it is in consequence made the type of a new genus. 



Horizon. Chouteau limestone. 



SHUMARDELLA OBSOLENS (Hall) 

 Plate XXVI, Figs. 1-9 



1855. Rhynchonella obscura-plicata Shumard, I and II, Rep. Geol. Surv. 



Mo., p. 218. (Name only.) 

 1855. Rhynchonella Missouriensis Shumard, I and II Hep. Geol. Surv. 



Mo., p. 204, pi. C, fig. 5a (not figs. 5b-c). 

 1860. Rhynchonella (Eatonia] obsolens Hall, 13th Rep. N. Y. State Cab. 



Nat. Hist., p. 111. 

 1866. Rhynchonella missouriensis Meek and Worthen, Geol. Surv. 111., 



vol. 2, p. 153, pi. 14, figs. 4a-b. 

 1910. Shumardella obsolescens Weller, Bull. Geol. Soc. Am., vol. 21, 



p. 513, fig. 15. 



Description. Shell below medium size, nearly smooth, subglobular in 

 form, wider than long, the greatest width near the mid-length, posterior 

 lateral margins meeting at the beak in an obtuse angle, the lateral mar- 

 gins rounded, the anterior margin emarginate. The dimensions of two 

 perfect specimens are: length 15.9 mm. and 14.4 mm., width 16.7 mm. 

 and 16.4 mm., thickness 13.7 mm. and 12.8 mm., width of sinus in front 

 11 mm. and 9.5 mm. 



Pedicle valve shallow, convex on the umbo, the surface gently convex 

 from the umbo to the antero-lateral margins, curving strongly to the 

 postero-lateral margins, the curvature becoming more abrupt towards the 

 beak, from the beak to the front margin along the median line the 

 curvature of the surface is a subsemicircle ; mesial sinus obsolete in the 

 umbonal region, originating as a broad, shallow, rounded depression post- 

 erior to the middle of the shell and becoming rapidly deeper anteriorly, 

 in front it is produced in a broad, elongate, terminally rounded lingual 

 extension which retains its concavity to the end giving to the shell its 

 emarginate outline in front ; beak small, incurved, nearly in contact with 

 the umbo of the opposite valve ; the delthyrium nearly filled by the beak 

 of the opposite valve, the deltidial plates obscure and not observed in 

 any of the specimens studied, the foramen small; plications nearly obso- 



