NIAGARA GROUP. 
165 
559. 4. FENESTELLA TENUICEPS (n. sp.). 
Px.. XL D. Fig. 2 a - h. 
Fenestella priscal Lonsdale, page 50 of this volume. 
Compare F. prisca, Lonsdale : Murchison, Sil. System, pag. 678, pi. 15, figs. 15, 15 a, 6, c, 
and 18, 18 a, 6, c. 
Frond cyathiform, much expanded, attached by a short cylindrical process to the spreading 
radicles below j 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 b 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 b. A portion of the last enlarged, showing the cells where the edges of the branches are 
worn. 
