PRODUCTUS 113 



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1892. Productus mesialis Hall and Clarke, Pal. N. Y., vol. 8, pt. 1, pi. 18, 

 figs. 9-10. 



Description. Shell of medium size or smaller, wider than long, the 

 greatest width along the hinge-line. The dimensions of a pedicle valve, 

 one of the co-types, are : length from hinge-line to front margin 20.2 mm., 

 length from umbonal region to front margin 23 mm., width along hinge- 

 line 31 mm., convexity 18 mm. 



Pedicle valve gibbous, more abruptly curved or subgeniculate near the 

 middle, the lateral slopes dropping abruptly to the margins, the umbonal 

 region rather broadly pointed and moderately protuberant beyond the 

 hinge-line, towards the cardinal extremities the surface is compressed and 

 extended into auriculations of moderate size which usually extend some- 

 what beyond the greatest width of the valve anteriorly and are convex 

 antero-posteriorly ; mesial sinus originating in the umbonal region, of mod- 

 erate width, somewhat variable in depth, rounded in the bottom ; the beak 

 pointed and incurved. Surface marked by rounded radiating costffi from 

 .5 mm to 1 mm. in width, which commonly increase by bifurcation upon 

 the posterior slope and less frequently or not at all upon the anterior 

 slope, sometimes becoming obsolescent towards the front and lateral mar- 

 gins; the posterior slope marked by concentric, wrinkle-like markings 

 of only moderate strength or rather faint. Spine bases rather sparsely 

 scattered over the anterior and lateral slopes with a row of about four 

 on each side of the beak along the cardinal margin, with sometimes a 

 few additional ones upon the cardinal auriculations. 



Brachial valve with a slight concavity towards the beak, beyond which 

 it is nearly flat throughout the visceral region, bending rather abruptly 

 into the produced anterior and lateral margins, the cardinal auricula- 

 tions only moderately differentiated from the general surface; towards 

 the front of the visceral surface the median portion of the valve is slightly 

 elevated in an obscure median fold which becomes much stronger upon 

 the anterior produced portion of the valve. Surface marked by radiating 

 costae similar to those of the opposite valve and by concentric, wrinkle- 

 like markings which are commonly rather faint. 



Remarks. This species is especially characterized by the subgeniculate 

 curvature of the pedicle valve, the well defined mesial sinus of the same 

 valve, and by the flattened visceral portion of the brachial valve. In 

 some respects the shell resembles P. burling tonensis, but it is smaller and 

 broader, with less regular curvature of the pedicle valve, and much flatter 

 visceral portion of the brachial valve. The depth of the mesial sinus of 

 the pedicle valve is a somewhat variable character, in some individuals 

 being almost obsolete. 



Horizon. Keokuk limestone. 



