346 MISSISSIPPIAN BRACHIOPODA 



cardinal extremities; mesial fold sharply defined at the beak, becoming 

 strongly elevated in front, marked by a deep, angular, median groove, 

 which gives it a distinctly bifurcate appearance, on each lateral slope of 

 the fold is a single, much weaker plication; lateral slopes of the valve 

 marked by plications similar in form and number to those of the opposite 

 valve. 



When perfectly preserved the surface of the entire shell is marked by 

 minute raised concentric and longitudinal lines which give it a finely re- 

 ticulate appearance when examined with a magnifying glass, the surface 

 is also marked by stronger concentric lines of growth which are some- 

 times crowded anteriorly. 



Remarks. In general form and size this species resembles 8. pellaensis, 

 but it does not exhibit so great variation in outline as that species. It 

 can be easily distinguished by its smaller number and much more angular 

 plications, as well as by the conspicuously bifurcate median fold of the 

 brachial valve. The species may be distinguished from immature exam- 

 ples of S. increbescens, with which it is sometimes associated, in much the 

 same way as from 8. pellaensis. 



Horizon. Chester group. 



SPIRIFER BIPURCATUS Hall 

 Plate XLVII, Figs. 6-16 



1856. Spirifer bifurcata, Hall, Trans. Albany Inst,, vol. 4. p. 8. 

 1866. Spirifer Leidyi var. Merimacensis Swallow, Trans. St. Louis Acad. 

 Sci., vol. 2, p. 410. 



1882. Spirifera bifurcata Whitfield, Bull. Am. Mus. Nat. Hist., vol. 1, 



p. 47, pi. 6, figs. 13-15. 



1883. Spirifera bifurcata Hall, 12th Kep. Geol. Surv. Ind., p. 326, pi. 29, 



figs. 13-15. 

 1895. Spirifer keokuk var. ? Hall and Clarke, Pal. N. Y., vol. 8, pt. 2, pi. 



37, figs. 13-15. 

 1906. Spirifer bifurcatus Beede, 30th Rep. Geol. Surv. Ind., p. 1314, pi 



22, figs 13-15 



Description. Shell usually below medium size, wider than long or occa- 

 sionally with the length and width subequal, greatest width usually along 

 the hinge-line, cardinal extremities usually nearly rectangular, sometimes 

 a little rounded. The dimensions of two individuals are : length 23 mm., 

 and 12.8 mm., width 29 mm. and 17 mm., thickness 17.5 mm. and 9.5 mm. 



Pedicle valve with its greatest convexity posterior to the middle, the 

 surface sloping abruptly from the umbonal region to the cardinal margin, 

 more gently to the antero-lateral margins, not at all or but slightly com- 

 pressed towards the cardinal extremities; beak pointed and incurved; 

 cardinal area concave, becoming more curved towards the beak, 2 to 4 

 mm. high, the inferior portion sloping posteriorly at an angle of about 



