Greville, on Neiv Diatoms. 
77 
or slightly convex, and the angles rounded. In all the speci- 
mens 1 have seen they are straight or very nearly so, but 
other valves in my possession have the sides decidedly con- 
vex, along with a generally distinct aspect at once appreciable 
by the eye, but difficult to convey in words. Among other 
characters, the value of which I do not at present venture to 
estimate, is the slightly produced angle in combination with 
the more or less convexity of the side, as seen in the present 
and following species. This feature has not been observed in 
T, margaritaceum, and may eventually be found to facilitate 
the diagnosis of these most perplexing diatoms. 
We have much pleasure in dedicating this fine species to 
Mr. Harrison, of Hull, who has devoted much attention to the 
microscopical investigation of the Diatomacea. 
Triceratium giganteum, n. sp., Grev. — Large. Valve with 
slightly convex sides, and rounded, somewhat produced, 
angles ; pearly granules, forming a marginal band of radiat- 
ing lines ; central space filled with minute, scattered spines. 
Distance between the angles -0066". (Fig. 10.) 
Hab. Barbadoes deposit; exceedingly rare; Christopher 
Johnson, Esq., George Norman, Esq. 
Scarcely less splendid than the preceding, and more 
remarkable on account of the singular spinulose, central 
surface. It is a robust species, with large, round, somewhat 
flattened, granules, and a very strong margin. For the 
specimen in my cabinet, from which my drawing was made, 
I am indebted to the kindness of my friend, Mrs. Bury. The 
only other frustule hitherto discovered, so far as I know, is 
in Mr. George Norman's collection. 
Amphitetras. 
Amphitetras minutaj n. sp., Grev. — Minute. Valve with 
deeply concave sides and rounded angles; lines of very 
minute puncta, radiating from the centre to every part of the 
circumference. Distance between the angles -0014". (Fig. 
11.) 
Hab. Nottingham deposit, Maryland, United States. 
I have seen several frustules of this inconspicuous little 
diatom, which is extremely liable to be overlooked. All are 
four- angled, and I venture to place it provisionally in the 
present genus. 
