GrevillEj on New Diaioms. 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 vahigs 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 minuta, 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. 



