346 Systematic Paleontology 



two-thirds the distance from the front to the beak ; beak incurved over that 

 of the opposite valve. The median portion of the valve is elevated in some 

 individuals to form a fairly well-defined fold which extends from a short 

 distance anterior to the umbo to the front, and contains several incon- 

 spicuous plications, while in other individuals the fold is very faint. On 

 each side of the median fold there are two to three simple plications which 

 are most prominent at the front and become obsolete from the center of 

 the shell to the beak, leaving this portion of the shell smooth. Dorsal 

 valve transversely subelliptical in outline, convex, greatest convexity just 

 anterior to the umbo ; much less convex than the ventral valve. It pos- 

 sesses a more or less depressed mesial sinus which never extends posterior 

 to the center of the shell, and which is bounded by two rounded plications. 

 One or two plications occur on each side of the sinus. Both valves are 

 marked by inconspicuous concentric lines of growth. Dimensions: 17 

 mm. long and 19 mm. wide. 



This species closely resembles Gypidula angulata Weller from the 

 Coeymans of New Jersey from which it differs as follows : The median 

 portion of the shell is elevated into a fold which is rarely sharply defined 

 and seldom extends farther than the center of the shell. The plications 

 on the fold are rounded, never subangular. More than one plication occurs 

 on each side of the fold and sinus. 



Occurrence. — Helderbekg Formation, Keyser Member. Devil's 

 Backbone, Maryland; Keyser, West Virginia. 



Collection. — U. S. National Museum. 



[Maynard.] 



Gypidula subglobosa n. sp. 

 Plate LXII, Figs. 20-22 

 Description. — Shell subglobose, both valves of almost equal convexity; 

 length and breadth about equal. Ventral valve convex, gibbous, becoming 

 ventricose in the umbonal region ; beak prominent, slightly incurved oppo- 

 site the beak of the dorsal valve. Dorsal valve of almost equal convexity 

 with the ventral valve, gibbous in the umbonal region but not ventricose ; 

 beak slightly incurved and opposite the beak of the ventral valve. Two 

 simple rounded plications form an undefined fold in the ventral valve 



