WALLICH, ON TRICERATIUM. 247 
both in Mr. Roper’s and Mr. Brightwell’s figures, to be sub- 
marginal. 
Mr. Brightwell again figures several varieties of 7. armatum. 
The first being the only one that in the least resembles the 
St. Helena form. But his figure is represented as being the 
front view of that described by Mr. Roper, which has already 
been shown to be distinct, and is clearly proved to be so, 
from the other figure given of a front view of a four-sided 
specimen, in which the cells are circular; the connecting 
membrane is marked as in Amphitetras, and its margin ex- 
hibits a bold fimbriated border. 
The next form to which I shall refer is a very large and 
beautiful Triceratium, obtained by dredging at St. Helena, in 
form thirty-five to forty fathoms, and to which I propose to 
give the specific name of T. fimbriatum. 
Its characters are as follows : 
Frustule three-, rarely four-sided; sides convex; angles 
furnished with short cornua; cells large, hexagonal; mar- 
ginal border between horns furnished with a series of two- 
lobed flabelliform and pedunculated fimbriz; connecting 
membrane marked with diamond-shaped striation. 
The circlet of remarkable fimbrize at once serves to distin- 
guish this species. These arise from the outer edge of the 
marginal row of cells, by delicate pedicles, which immediately 
expand into broad filabelliform discs, having their flat 
surfaces parallel to the margin of the valve, and divided 
down their centre by a deep notch. These fimbriz are very 
similar in outline to the architectural decoration called “ Greek 
tiles,’ which are small separate mouldings, placed at intervals 
on the cornice of a building, along the side of the roof, and 
serve to conceal the ridge formed by the overlapping of the 
roof tiles. In some frustules there exist also, at each angle 
of each hexagonal cell, minute dot-like processes (recently 
figured by Mr. Roper as existing in Hupodiscus tesselatus), 
which, seen in profile when a portion of the valve is broken 
up, prove to be minute discs of similar character to the fimbriz 
just alluded to. 
This species has, however, another peculiarity which would 
render the specific name of favus especially applicable, had 
it not been already assigned to the typical species, to denote 
the similarity, in superficial aspect only, of the hexagonal 
markings to the honey-comb. Each hexagon in the St. 
Helen form being, not merely a simple depression dependent 
on the mode in which the siliceous element is secreted by the 
inner cell-membrane on its own surface, but a deep hollow 
cell, with perpendicular sides, of sufficient depth to be 
