SPIRIFERINA 291 



is clearly shown in the chert specimens by the little siliceous pillars 

 which join the two surfaces of the cavity left by the solution of the 

 shell, these pillars being the casts of the perforations which penetrated the 

 shell through its entire thickness, unless a thin epidermal imperforate 

 layer was removed before the shells were fossilized. 

 Horizon. Residual chert, probably Keokuk age. 



Genus SPIRIFERINA d'Orbigny 



Description. Shells usually small, spiriferoid in form, transverse, the 

 greatest width usually along the hinge-line and the cardinal extremities 

 acutely angular ; mesial fold and sinus well developed, either non-plicate 

 or with a single median plication ; lateral slopes of the valves covered 

 with simple plications, and the whole shell marked by sublamellose, con- 

 centric lines of growth. Pedicle valve with a moderately high, arched 

 cardinal area having an open delthyrium ; internally the dental plates are 

 well developed and continue to the inner surface of the valve along the 

 lateral margins of the muscular scar, between them a well developed 

 median septum is present which extends further anteriorly than the 

 dental lamella? and may reach beyond the center of the valve. In the 

 brachial valve the spiral cones are directed laterally as in Spirifer, the 

 primary lamelke being joined by a simple transverse or subacute jugum. 

 Shell structure strongly punctate throughout. 



Remarks. Externally the members of this genus are not unlike some 

 small species of the genus Spirifer, the essential characters by which 

 they are differentiated from that genus being the punctate shell structure 

 and the very strong median septum in the pedicle valve. From the genus 

 Delthyris, Spiriferina differs essentially only in the punctate shell struc- 

 ture, and a number of Mississippian species which have been commonly 

 referred to Spiriferina heretofore, but in which no punctate shell structure 

 has been detected, are transferred to Delthyris in the present report. 



SPIRIFERINA SUBTEXTA White 

 Plate XXXVI, Figs. 35-40 



1862. Spiriferina f subtexta White, Proc. Boston Soc. Nat. Hist., vol. 9, 



p. 25. 

 1901. Spiriferina subtexta Weller, Trans. St. Louis Acad. Sci., vol. 11, 



p. 199, pi. 20, figs. 5-6. 

 1909. Spiriferina subtexta Weller, Bull. Geol. Soc. Am., vol. 20, p. 309, 



pi. 13, figs. 16-19. 



Description. Shell small, wider than long, the hinge-line usually a little 

 shorter than the greatest width of the shell, and the cardinal extremities 

 usually abruptly and narrowly rounded. The dimensions of a nearly 

 perfect pedicle valve are : length from front to beak 10.5 mm., width 



