394 Systematic Paleontology 



portion as the ventral valve, the convexity being uniform, greatest con- 

 vexity about one-third the distance from the beak towards the anterior; 

 beak small, pointed, not prominent and only slightly incurved. The 

 anterior margin is abruptly inflected to correspond to the abrupt de- 

 flection of the anterior margin of the ventral valve. Surface marked by 

 eight simple rounded plications on each valve which radiate from the beak 

 and vs^hich are more prominent along and near the median portion of both 

 valves. In the ventral valve the three medial plications form an undefined 

 mesial elevation. There is, however, no indication of a sinus on the 

 brachial valve. Both valves are marked by well-defined concentric im- 

 bricating lamellae which give to the shell a distinctly rugose appearance. 



Dimensions : 13 mm. long and 16 mm. wide. 



This shell most closely resembles A trypa rugosa of the Niagara. It is 

 not as convex as that species, the plications are not as numerous and do not 

 bifurcate, and it has no marked mesial depression on the dorsal valve. 



Occurrence. — Helderberg Formatiox, Keyser jMember. Keyser, 

 West Virginia; Cash Valley, Maryland. 



Collection. — U. S. National Museum. 



[Maynard.] 



Genus ATRYPINA Hall and Clarke 

 AtRYPINA IMBRICATA (Hall) 



Plate LXVIII, Figs. 4-6 



Leptocoelia imbricata Hall, 1857, Tenth Ann. Rept. N. Y. State Cab. Nat. Hist., 



p. 108. 

 Leptocwlia imbricata Hall, 1859, Nat. Hist. N. Y., Pal., vol. iii, p. 246, pi. 



xxxviii, figs. 8-13. 

 Atrypina imbricata Hall and Clarke, 1893, ibidem, vol. viii, pt. ii, pi. liii, 



figs. 5, 6, 8-10. 



Description.- — "Shell longitudinally semielliptical or suborbicular : 

 ventral valve convex, most prominent along the middle, and sloping 

 laterally ; beak small, incurved at the apex and perforated by a very small 

 round aperture, one side of which is formed by the deltidium : dorsal valve 

 flattened or depressed convex; beak scarcely elevated above the hinge; 

 hinge sloping from the beaks at an angle of about 150°, rounded at the 



