PTYCHOSPIRA 435 



the type specimen, although where the beak of the pedicle valve is frac- 

 tured the character of the dental lamellae and median septum can be 

 observed. 



Horizon. Salem limestone. 



Family RHYNCHOSPIRID^ 

 Genus PTYCIIOSPIRA Hall 



Description. Shells small, biconvex, subcircular in outline, the hinge- 

 line short, the valves bearing a few, strong, subangular, simple plications. 

 Pedicle valve with an erect or moderately incurved beak perforated by a 

 circular foramen, the cardinal area small and sharply defined along its 

 lateral margins, the delthyrium more or less completely closed by the 

 pseudodeltidium, not including the foramen. The brachial valve with 

 convexity about equal to that of the pedicle, internally the hinge-plate is 

 supported by a well-defined median septum, and the spires are constituted 

 of but four or five volutions. The epidermal layer of the shell is finely 

 pitted but the punetations do not continue into the deeper layers of the 

 shell. 



Remarks. This genus is easily recognized by reason of its subequally 

 convex valves, its subcircular outline, and its few but strong plications. 

 It has only been observed in faunas of Kinderhook or lower Burlington 

 age. 



PTYCHOSPIRA SEXPLICATA (White and Whitfield) 

 Plate LXXVI, Figs. 35-46 



1862. Retzia sexplicata White and Whitfield, Proc. Boston Soc. Nat. Hist., 



vol. 8, p. 294. 

 3892. Retzia plicata Miller, Adv. Sheets 18th Rep. Geol. Surv. Ind., p. 62, 



pi. 9, figs. 29-31. 

 1894. Retzia plicata Miller, 18th Rep. Geol. Surv. Ind., p. 316, pi. 9, figs. 



29-31. 



1894. Ptychospira sexplicata Hall and Clarke, Int. to Study of Brach., pt. 



2, pi. 36, figs. 22-23. 



1895. Ptychospira (cf.) sexplicata Hall & Clarke, Pal. N. Y., vol. 8, pt. 2, 



pi. 50, figs. 13-14. 

 1895. Ptychospira sexplicata Hall and Clarke, Pal. N. Y., vol. 8, pt. 2, 



pi. 83, fig. 28. 

 1900. Retzia f raricosta Rowley, Am. Geol., vol. 25, p. 266, pi. 5, figs. 



34-37. 



1904. Ptychospira sexplicata Greger, Am. Geol., vol. 33, p. 15. 

 1909. Ptychospira sexplicata Weller, Bull. Geol. Soc. Am., vol. 20, pi. 315, 



pi. 14, fig. 11. 



Description. Shell small, subcircular in outline, usually a little wider 

 than long but sometimes longer than wide, sublenticular to subglobular 

 in form, the valves subequally convex, the greatest width near the mid- 



