416 MISSISSIPPI AN BRACHIOPODA 



from 6 to 8 broad, depressed, rounded, more or less obscure, simple plica- 

 tions, which become obsolete towards the cardinal extremities ; mesial 

 sinus originating at the beak, becoming broad and moderately deep 

 towards the front, it is nearly flat in the bottom or more often is occu- 

 pied by a broad, depressed convex region which can scarcely be called 

 a plication, although in the narrower examples it assumes more the form 

 of a plication, the lateral slopes of the sinus are convex and pass without 

 demarcation into the surface of the lateral slopes of the valve. 



Brachial valve a little less convex than the pedicle, its greatest con- 

 vexity near the middle, the convexity extending out to the cardinal ex- 

 tremities; the beak slightly incurved and extending a little beyond the 

 hinge-line; the mesial fold narrow at the beak, becoming broad and in 

 large examples very high in front where it is rounded and free from 

 plications; lateral slopes bearing plications similar in form and number 

 to those of the opposite valve. 



Shell structure punctate ; the minute surface markings not preserved, 

 but there are concentric lines of growth which are sometimes strongly 

 developed and crowded towards the anterior margin. 



In the internal casts of the shell preserved in residual cherts, the 

 dental lamella? of the pedicle valve are seen to be elongate, extending 

 nearly one-third the total length of the valve from the beak towards 

 the anterior margin, and diverging only to a moderate degree; the mus- 

 cular impressions in the same specimens are scarcely defined. 



Remarks, This species is so distinctly marked that it cannot be con- 

 fused with any other form. In the rounding of the lateral slopes of the 

 pedicle valve into the false cardinal area it resembles Spirifer sub- 

 cardiiformis, but it attains a larger size than that species and differs from 

 it in the lack of any line of sharp demarcation between the false and the 

 true cardinal area, as well as in the lack of plications in the fold and 

 sinus, and in the punctate structure of the shell. In general form the 

 species most closely resembles S. plena, but the cardinal area of the 

 pedicle valve of that species is much larger and more denned, and the 

 plications of the shell are narrower and much more numerous. 



Horizon. Keokuk limestone. 



SPIRIFERELLA SCHUCHERTI (Rowley) 



Plate XXXV, Figs. 59-63 



1900. Spirifer schucherti Rowley, Am. Geol., vol. 25, p. 261, pi. 5, figs. 

 15-17, 59. 



Description. Shell small, suborbicular in outline, a little wider than 

 long, the greatest width at about the mid-length of the shell, the hinge- 

 line shorter than the greatest width, the cardinal extremities rounded. 

 The dimensions of a nearly perfect specimen, a metatype, are : length of 

 pedicle valve 8.9 mm., length of brachial valve 7.5 mm., greatest width 



