98 MISSISSIPPIAN BRACHIOPODA 



Remarks. The characters commonly designated as the essential fea- 

 tures of Productella as distinguished from Productus, viz., the narrow 

 cardinal area and weakly developed articulating hinge-teeth and sockets, 

 can rarely be distinguished upon the specimens, and in actual practice the 

 type of surface markings of the shell is commonly relied upon for the 

 recognition of the genus. In the genus Productus these markings are al- 

 ways clear cut and well defined, while in Productella there is commonly 

 an indefiniteness about them which is in strong contrast to the condition 

 in Productus. In Productella the radiating markings are commonly more 

 or less discontinuous, often elongate, node-like elevations of the surface, 

 rather than distinct, continuous costae. The surface spines are also com- 

 monly arranged in a more or less irregular manner in Productella, while 

 in Productus each species exhibits a fairly definite arrangement of its 

 surface spines. 



PRODUCTELLA CONCENTRICA (Hall) 

 Plate XIX, Figs. 22-34 



1857. Productus concentricus Hall, 10th Rep. N. Y. State Cab. Nat. Hist., 



p. 180. 



1858. Productus shumardianus Hall, Geol. Iowa, vol. 1, pt. 2, p. 499, pi. 7, 



fig. 1 (not pi. 3, fig. 9). 

 1858. Productus concentricus Hall, Geol. Iowa, vol. 1, pt. 2, p. 517, pi. 7, 



ng. 3. 

 1860. Productus Cooperensis Swallow, Trans. St. Louis Acad. Sci., vol. 1, 



p. 640. 



1862. Producta concentrica Winchell, Proc. Acad. Nat. Sci. Phil., p. 411. 

 1865. Producta concentrica Winchell, Proc. Acad. Nat. Sci. Phil., p. 114. 

 1865. Producta Cooperensis Winchell, Proc. Acad. Nat. Sci. Phil., p. 115. 

 1867. Productella shumardiana Hall, Pal. N. Y., vol. 4, p. 157, pi. 23, 



figs. 6-8. 



1869. Producta concentrica Winchell, Safford's Geol. Tenn., p. 443. 



1870. Producta concentrica Winchell, Proc. Am. Phil. Soc., vol. 11, p. 249. 

 1888. Productus concentricus Herrick, Bull. Sci. Lab. Denison Univ., vol. 



3, p. 33, pi. 6, fig. 16. 



1888. Productus (Productella} shumardianus Herrick, Bull. Sci. Lab. Den- 

 ison Univ., vol. 3, p. 32, pi. 7, fig. 18 ; pi. 12, figs. 6, 43. 

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



pi. 17, fig. 7. 

 1899. Productella Cooperensis Girty, Mon. U. S. Geol. Survey No. 32, p. 



528, pi. 68, figs. 8a-c, 9a-b. 

 1901. Productella concentrica Weller, Trans. St. Louis Acad. Sci., vol. 11, 



p. 184, pi. 16, figs. 12-14. 



Description. Shell below medium size, length and width sub-equal, or 

 a little longer than wide, the hinge-line shorter than the greatest width, 

 the cardinal extremities angular. The dimensions of a nearly complete 

 specimen are : length from hinge-line to front margin 13 mm., length from 

 umbonal region of pedicle valve to front margin 15 mm., greatest width 

 15 mm., length of hinge-line 11 mm., convexity of pedicle valve 10.2 mm. 



