410 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.56. 



Some specimens do not show a further development but the 

 greater number show that the median pUcation in the sinus has 

 bifurcated and a seventh plication is intercalated on the fold in a 

 median position. 



There are 9 to 13 somewhat round plications on the lateral slopes. 

 The slopes of the pedicle valve are almost flat or slightly concave; 

 those of the brachial valve are slightly convex. The fold is low and 

 broad and the sinus is shallow. The characteristic granular markings 

 of the surface are unusually distinct. 



Measurements. — 27.4 mm. along the hinge, 24 mm. wide across the 

 middle, 14.4 mm. high, 10 mm. thick, 9 mm. sinal width, 4.2 mm 

 sinal depth, 1.2 mm. fold height. 



Occurrence. — Trenton group. Hermitage limestone; Payton's 

 Creek, 6 miles northwest of Carthage, and Auburn, Tennessee. 



Cotypes.— Cat. Nos. 66100, 66101, U.S.N.M. 



3. TRIPLICATE GROUP. 



a. LOW FOLD STJBGROTTP. 



PLATYSTROPHIA EXTENSA, new species. 



Plate 42, figs. 39-il. 



Associated with PlatsystropMa Tiermitagensis is a species with the 

 same general physiognomy. Its distinguishing characteristic is the 

 plication pattern of the fold and sinus which places it with the 

 Triplicate group. 



The hinge line varies from nearly as great to greater than the width 

 of the shell across the middle. The valves are subequally convex. 

 The slopes of the brachial valve are flattish, with a tendency to 

 become concave; those of the pedicle valve are convex. There are 

 9 to 13 round plications on the lateral slopes, which are separated by 

 furrows of their own width. 



Two specimens were found in the Decorah shale near Fennemore, 

 Tennessee, which are referred to this species. However, they have 

 proportionally longer hinge lines and more ventricose forms than the 

 Trenton specimens. 



A specimen measures 21 mm. along the hinge, 20.5 mm. wide across 

 the middle, 12 mm. high, 8.8 mm. thick, 7.5 mm. sinal width, 2.7 mm. 

 sinal depth, 1 mm. fold height. 



Occurrence. — Trenton group: Hermitage limestone at Hartsville 

 and Auburn, Tennessee. Black River group: Decorah shale 4^ 

 miles north of Fennimore, Wisconsin. 



Ootypes. —C&t. Nos. 66102, 66103, U.S.N.M. 



PLATYSTROPHIA ELEGANTULA, new species. 



Plate 43, figs. 44-47. 

 The types of this species were found m the Bigby limestone of the 

 Trenton group at Frankfort, Kentucky, and vicinity. It is common 



