100 MISSISSIPPIAN BRACHIOPODA 



more strongly convex pedicle valve, with much smaller cardinal auricula- 

 tions, the surface markings of the two shells are also distinctly different. 

 The species agrees more closely with the middle Devonian P. spinulicosta, 

 and it has sometimes been suggested that these two forms are identical, 

 both having the same general form, size, and surface markings. It is pos- 

 sible that this interpretation should be held, but for the present these 

 Mississippian shells will be considered as distinct from the middle Devon- 

 ian form, although an entirely satisfactory method of distinguishing them 

 cannot be pointed out. 



The species exhibits considerable variation in size and in the strength 

 of the development of the surface markings. It sometimes attains a 

 length and width of 20 mm. or more. The variation in the surface mark- 

 ings is more conspicuous in the longitudinal features, although the con- 

 centric folds of the visceral portion of the pedicle valve are sometimes 

 almost obsolete; the surface features of the brachial valve are usually 

 more constant than those of the pedicle valve. 



Horizon. Kinderhook. 



PRODUCTELLA PYXIDATA Hall 

 Plate XIX, Figs. 1-21 



1858. Productus 2>yxidatus Hall, Geol. Iowa, vol. 1, pt. 2, p. 498, pi. 3, 



figs. 8a-e. 

 1858. Productus shumardianus Hall, Geol. Iowa, vol. 1, pt. 2, p. 499, pi. 3, 



fig. 9 (not pi. 7, fig. 1). 

 1883. Productella pyxidata Hall, Rep. N. Y. State Geol. for 1882, pi. (17) 



48, fig. 34. 

 1892. Productella pyxidata, Hall and Clarke, Int. to Study of Brach., pt. 1, 



pi. 21, figs. 20, 23. 

 1892. Productella pyxidata Hall and Clarke, Pal. N. Y., vol. 8, pt. 1, pi. 



17, fig. 34, pi. 17 A, fig. 14. 

 1894. Productella pyxidata Keyes, Mo. Geol. Surv., vol. 5, p. 52, pi. 



38, figs. 4a-d. 

 1908. Productella pyxidata Rowley, Mo. Bureau Geol. and Mines, vol. 8, 



2nd ser., p. 77, pi. 17, figs. 5, 30-36. 



Description. Shell usually below medium size, and wider than long, sub- 

 semielliptical in outline, the hinge-line a little shorter than the greatest 

 width, the cardinal extremities rounded. The dimensions of a nearly 

 complete specimen of average size are: length from hinge-line to front 

 margin 14.4 mm., length from umbonal region of pedicle valve to front 

 margin 16.4 mm., greatest width 19.1 mm., length of hinge-line 17 mm., 

 convexity of pedicle valve 17 mm., depth of visceral cavity 4.6 mm. 



Pedicle valve moderately convex, the greatest convexity posterior to the 

 middle, the umbonal region projecting somewhat beyond the hinge-line, 

 the surface curving abruptly from the umbonal region to the cardinal 

 margin, curving less abruptly to the lateral margins and more gently to 



