76 
GrevillEj on New Diatoms, 
Hab. Nottingham deposit, Maryland, U.S. 
Not rare, yet I cannot refer it to any described species. 
It is a neat and brilliant little diatom. The puncta or minute 
granules are rather distant, the largest being those imme- 
diately external to the raised centre ; in the angles they 
again become smaller. The angles themselves are frequently, 
though not invariably, slightly dilated, as in fig. 7, and are 
thickened in substance, but no distinct pseudo-nodule is 
perceptible. 
Triceratium obscurum, n. sp., Grev. — Small. Valve thin 
and delicate, with nearly straignt sides and rounded angles ; 
puncta equal, very minute, radiating in straight lines. Dis- 
tance between the angles -0024". (Fig. 8.) 
Hab. South Naparima deposit, Trinidad. 
Contour exactly resembling that of jT. condecorum, but the 
radiating lines of puncta are perfectly straight. The puncta 
are also somewhat more minute. 
Triceratium Harris onianum, n. sp., Norman and Grev. — 
Large. Valve with convex sides and slightly produced, 
rounded angles ; pearly granules forming a marginal band of 
radiating rows, and thinly scattered over the ample central 
space, in which is a conspicuous network of large, elongated, 
radiating cellules, sending down lines between the rows of 
granules to the margin ; rows 4 in '001". Distance between 
the angles -0070". (Fig. 9.) 
Hab. Barbadoes deposit (Springfield Estate); exceedingly 
rare ; George Norman, Esq. 
A truly splendid diatom, belonging to a small, very natural 
group, and, as is frequent in such cases, extremely difficult to 
define satisfactorily. It may be, indeed, that most of them 
constitute but one species ; and if so, it becomes all the more 
necessary that they should be carefully figured and described. 
This I hope to be able to do in a future series. T. mar^ 
garitacevm, described by Ralfs in the last edition of ^Pritchard's 
Infusoria,^ is the only one hitherto published, and, as the first 
known, may stand as the type. It is comparatively a small 
species, the distance between the angles being only about 
•0030", often less. All the members of the group, however, 
possess the same structural arrangement, the central portion of 
the valve being composed of large, radiating, elongated cellules, 
which towards the margin become smaller and quadrangular, 
each of the quadrangular cellules containing a round, pearly 
granule. In none of the species are these characters seen so 
conspicuously as in our new T. Harrisonianum. The outline 
of the valve in these species varies considerably. According 
to R alfs, the sides of the valve in T. margaritaceum are straight 
