360 MISSISSIPPIAN BRACHIOPODA 



Remarks. In its small size and fine costee, this species agrees most 

 closely with E. jersey ensis, but it differs externally from that species in 

 being proportionally more elongate and in the entire absence or very 

 slight development of a mesial sinus in the brachial valve. Internally 

 this species is totally different from E. jerseyensis in the characters of 

 the cardinal process and socket plates, the cardinal process being exceed- 

 ingly large and prominent, while in that species it is small. The median 

 ridge along the inner surface of the brachial valve is much more strongly 

 developed than in any other member of the genus here described. 



Horizon. Chouteau limestone. 



RHIPIDOMEI^LA DUBIA (Hall) 

 Plate XX, Figs. 22-26 ; Plate LXXXIII, Figs. 9-10 



1856. Orthis dubia Hall, Trans. Albany Inst., vol. 4, p. 12. 

 1863. Orthis Cooperensis Swallow, Trans. St. Louis Acad. Sci., vol. 2, 

 p. 82. 



1882. Orthis dubia Whitfield, Bull. Am. Mus. Ntat. Hist., vol. 1, p. 45, 



pi. 6, figs. 1-5. 



1883. Orthis dubia Hall, 12th Ann. Rep. Geol. Surv. Ind., p. 324, pi. 29, 



figs. 1-5. 

 1892. Orthis dubia Hall and Clarke, Int. to Study of Brach., pt. 1, pi. 12, 



figs. 10-13. 

 1892. Orthis dubia Hall and Clarke, Pal. N. Y., vol. 8, pt 1, pi. 6A, 



figs. 18-22. 



1894. Orthis dubia Keyes, Mo. Geol. Surv., vol. 5, p. 64. 

 1906. Rhipidcmella dubia Beede, 30th Ann. Rep. G-eol. Surv. Ind., p. 



1303, pi. 22, figs. 1-4. 



Description. Shell small, usually a little longer than wide, subovate in 

 outline, the greatest width usually anterior to the middle, the anterior 

 margin nearly straight or slightly convex in the middle, sometimes 

 slightly emarginate, the antero-lateral margins regularly rounded, the 

 postero-lateral margins gently convex, becoming straighter as they ap- 

 proach the beak where they meet usually in an angle a little less than a 

 right angle, the hinge-line very short ; the two valves subequally convex. 

 The dimensions of two individuals are: length of pedicle valve 14 mm. 

 and 11.7 mm., length of brachial valve 13 mm. and 11 mm., greatest width 

 13.7 mm and 11.9 mm., thickness 8 mm. and 6.6 mm. 



Pedicle valve flattened medially in its posterior half, the surface curv- 

 ing abruptly to the posterio-lateral margins, often with a slight incurva- 

 ture to the cardinal extremities, and curving gently to the anterior and 

 antero-lateral margins; mesial sinus rather broad and rather shallow, 

 originating in or near the umbonal region, its lateral boundaries ill- 

 defined, sometimes rather abruptly deepened as it approaches the anterior 

 margin ; the beak compressed, slightly or only moderately incurved ; car- 



