270 
PALEONTOLOGY OF NEW YORK. 
this type of structure by the name Oriskania. The typical form is described 
in the Supplement to this Volume as Oriskania navicella. 
It is a fact of much interest that the centronellid type, with naviculoid shell, 
divided hinge-plate and simple brachidiurn, is reproduced in the Alpine Triassic 
faunas, in the genus (?) Nucleatula, Zugmayer,"^ a shell which evinces only an 
immaterial variation in the narrowness and fimbriation of the anterior plate of 
the brachidiurn. 
Before considering the later modifications of the centronellid type, we may 
refer to another form of exterior expression assumed by the same type. A 
small, hitherto unnoticed species occurring in the Upper Helderberg of the 
Province of Ontario, possesses a smooth exterior, terebratuliform outline, nar¬ 
row at the umbones and broad in the pallial region, with biconvex valves. 
While the detailed structure of the hinge-plate is yet unknown, the brachidiurn 
is similar to that of Renssel^ria and Centronella ; the anterior plate broader 
and less attenuate than in Renssel^ria and without its central, rod-like poste¬ 
rior extension, and also lacks the median ridge or thickening along the sym¬ 
physis of the lateral elements, which exists in Centronella. The form of the 
shell scarcely suggests the naviculoid contour of true Centronella, while it at 
once brings to mind some of the biconvex species that have heretofore been 
Fig. I8t. Fig. 185. Fig. 186. 
Selenella gracilis, sp. nov. 
Fig. 184. Outline profile of conjoined valves. 
Fig. 185. Preparation showing the form of the loop. 
Fig. 186. An oblique view; showing the upward curvature of the anterior plate. X 3. (C.) 
classed with that genus, e. g., Centronella Julia. From such of these whose inte¬ 
riors are known it differs notably, and it so evidently indicates a distinct stadium 
* See ISiTTNER, 13i*achiopo(len der Alpinen Trias; Abhaiidl. der K.-k. Geol. Reichstanst., vol. xiv, 
p. 208. 1890. 
