NIAGARA GROUP. 



165 



559. 4. FENESTELLA TENUICEPS (n. j?j9.). 



PL. XL D. Fig. 2a -h. 



Fenestella prisca! Lonsdale, page 50 of this volume. 



Compare F. prisca, Lonsdale : Murchison, Sil. System, pag. 678, pL 15, figs. 15, 15 a, b, c, 

 and 18, 18 a, 6, c. 



Frond cyatbifonn, much expanded, attached by a short cylindrical process to the spreading 

 radicles below ; external face presenting sharp thin-edged branches ; transverse bars not reach- 

 ing so high as the edges of the branches ; cells round, rather large, opening laterally, being 

 scarcely visible in looking upon the face of the frond (particularly near the base) ; cells four 

 rarely five in each fenestrule ; non-celluliferous side with oval fenestrules and striated surface. 



Fenestrules about seven in the space of three lines longitudinally, and nine or ten in the 

 same space transversely. 



I am unable to find any reliable characters for separating this species from the one in the 

 Clinton group, but a more perfect specimen in the Niagara shale proves that it can not be the 

 F. prisca of Lonsdale. The spreading form of the cup is very different from those figured 

 by this author (pi. 15, figs. 15, 15 a 6 c), and the cells are proportionally larger and more 

 prominent. 



In perfect specimens, the distinguishing features of this species are the sharp thin-edged 

 branches on the celluliferous side, and which scarcely show any signs of connecting bars. 

 Where they can be examined, the cells are round short tubes, the elevated edges projecting 

 almost laterally from the side of the branch. The upper or non-celluliferous face is marked by 

 short oval fenestrules, caused by the thickening of the dissepiments at their junction with the 

 branches. In perfect specimens, the branches are striated longitudinally on this side, but, from 

 weathering, they often appear granular. A partial abrasion of the celluliferous face often wears 

 away the thin edges of the branches, and they not only appear thicker than in their natural 

 state, but the cells lie close to the upper edge of the branches. The fenestrules in such speci- 

 mens are quadrangular, the dissepiments not expanding at their junction with the branches. 



The specimens found in the Clinton group are usually in such condition that the characters 

 are obscure, and the celluliferous face rarely if ever well preserved ; the specimen figured on 

 pi. 19, fig. 4 a, being the only one in which the cells have been observed at all. 



Fig. 2 a. The exterior surface of a large cyathiform frond which has been crushed. The point 

 of attachment to the root is well preserved. The surface is somewhat worn, so that 

 the pores are more distinctly visible in some parts than is usual in this species. 



Fig. 2 i. A portion of the last enlarged, showing the cells where the edges of the branches are 

 worn. 



