18 The Fauxa of the Keyseu Member of the 



and seldom extends farther than the center of the shell. The ])lications 

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

 on each side of the fold and sinus. 



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

 Backbone, Maryland ; Keyser, West Virginia. 



Collection.— U. S. Xational Museum. 



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 

 which is only conspicuous from the center of the valve to the front, and a 

 single plication on the brachial valve marks the center of an undefined 

 sinus, bordered by two inconspicuous plications near the front. The shell 

 is smooth on both valves from the center posteriorly. The slope from 

 the umbonal regions is abrupt towards the posterior and the posterior 

 region becomes concave ; the lateral margins rounded. The shell is marked 

 by faint concentric lines of growth. Dimensions of an average individual : 

 Length 12 mm. ; width 13 mm. 



This species is distinguished from Gypidula corriganensis by its some- 

 what more globose character, the almost equal convexity of the valves, and 

 by the prominent beaks which are almost equal and directly opposite each 

 other, and by the abrupt slope from the umbonal region to the posterior. 



Occurrence. — Helderberg Formation, Keyser Member, Cash Valley. 



Collection. — U. S. jSTational Museum. 



