BracuiopuDA. 855 
is shortened and constricted, forming a projecting cardinal 
spur. 
(Type, Oriskania navicella, Hall. Oriskany sandstone.) 
Subgenus Selenella, Hall. 1893. 
Shells with biconvex valves, smooth exterior, terebratuliform 
outline, narrow at the umbones and broad in the pallial region. 
The brachidium is similar to that of Renssptmria and Cern- 
TR NELLA; the anterior plate broader and less attenuate than in 
Fia. 466. Fie. 467. Fia. 468. 
Selenella gracilis, Hall. 
Fie. 466. Outline profile of conjoined valves. 
Fig. 467. Preparation showing the form of the loop. 
Fic. 468. An oblique view; showing the upward curvature of the anterior plate. x 8. 
RensseLazura and without the central, rod-like posterior extension, 
and also lacks the median ridge or thickening along the sym- 
physis of the lateral elements, which exists in CenrRonELLA. 
(Type, Selenella gracilis, Hall. Upper Helderberg group.) 
Subgenus Romingerina, Hall. 1893. 
(Plate 52, figs. 14, 15.) 
Shells, small biconvex; smooth valved. The median ridge 
on the anterior plate of the brachidium is elevated into a con- 
spicuous vertical lamella, extended both anteriorly and posteriorly, 
being in fact a double plate produced by the abrupt deflection of 
each lateral branch of the brachidium near the median line; 
union taking place along the upper edge, which almost reaches 
the inner surface of the pedicle-valve. 
The upper edge, where viewed from the side, is flatly roof- 
shaped, while the lower edge describes two convexities, the 
greater anterior, leaving a notch between them. The surfaces 
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