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 brachidium, 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 brachidium. 



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 brachidium 

 is similar to that of RENSSELiERiA and Centronella ; the anterior plate broader 

 and less attenuate than in RENSSELiERiA 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. 184. Fi<;. ISS. 



Selenella gracilis, sp. nov. 

 Fig. 184. Outline prollle of conjoineil v.^lves. 

 Fife'. 186. Preparation sliowinft the I'orm of the loop. 

 Fig. 18B. An oblique view; showing Ihe upward curvature of the anterior plate. X 3. 



FlO. 186. 



(C.) 



classed with that genus, e. g., Centronella Julia. From such of these whose inte- 

 riors are known it diifers notably, and it so evidently indicates a distinct stadium 



* See BiTTNER, Bi^aehiopodeii del- Alpiiien Trias; Abhandl. der K.-k. Geol. Reichstanst., vol. xiv, 

 p. 208. 1S90. 



