258 SYSTEMATIC PALEONTOLOGY 



somewhat shorter than, the greatest width of the shell. Surface marked 

 by fine, closely crowded, alternating striae, as in Rafinesquina alternata, 

 crossed by exceedingly delicate concentric lines and over the central flat 

 disc of each valve by more or less continuous zigzag undulations or 

 wrinkles. 



" Ventral valve depressed-convex over the greater portion of the shell 

 and more or less suddenly bent downward or geniculated along the 

 margin, especially anteriorly. Cardinal area wide, broadly triangular, 

 with a convex deltidium, wider than long, apically perforated by a rather 

 large pedicle opening, posteriorly excavated and completely occupied by 

 the chilidium of the other valve. Crenulated hinge teeth prominent and 

 supported by short dental plates, which are attached to the elevated outer 

 margin of the small, transversely oval muscular area. Within this area, 

 in the center of the mesial thickening, are placed the short and narrow 

 adductors, surrounded by the large diductors, and outside these, at the 

 base of the dental plates, are the distinct scars of the small adjusters. 

 Surface marked by delicate, crowded papillae, strongest in front of the 

 muscular area, and in the thin shells by the wrinkling of the outer surface. 



" Dorsal valve nearly flat, with the anterior margin more or less 

 reflexed downward. Cardinal area narrow, about one-third that of the 

 other valve, with a broad and strongly convex chilidium. Dental sockets 

 deep; crural plates slender, very bilobed, cordate cardinal process; in 

 front of this is a short, low septum separating the inconspicuous septa. 

 Just inside the outer margin of the valve is situated a prominent, rounded 

 ridge of the same nature as that in L. rhomboidalis." "Winchell and 

 Schuchert, 1893. 



Leptaena charlottae differs conspicuously from all other American 

 species of the genus in its zigzag, concentric surface corrugations. 



Occurrence. CHAMBERSBURG LIMESTONE ( Caryocystites bed). Fort 

 Loudon and Blue Spring, Franklin County, Pennsylvania. 



The type specimens were described from the Decorah shales division of 

 the Black River at Minneapolis and St. Paul, Minnesota. 



Collections. Maryland Geological Survey, TJ. S. National Museum. 



