10 
Greville_, on New Diatoms. 
with nearly straight sides and rounded angles^ which contain 
large hemispherical pseudo-nodules, having a nucleus of very 
minute puncta ; surface with 6 alternately raised and depressed 
radiating undulations ; structure composed of lines of minute 
puncta radiating from a small blank umbilicus ; margin 
coarsely striated. (Fig. 25.) 
Hab. Barbadoes deposit_, Cambridge estate; C. Johnson, 
Esq. ; very rare. 
This fine diatom belongs to a small group in which occur 
the remarkable undulations of the surface of the valve, already 
described in T. cancellatum. They exist also in T. insignej a 
fact which I overlooked in my description of that species, but 
which was detected by my acute friend Mr. Ralfs. It may 
be a question whether the present diatom be not a variety of 
the one last named, but after the examination of a series of 
specimens, I am under a very strong impression that the two, 
although very nearly related, are really distinct. Our present 
species is larger, the sides always nearly straight, and the 
angles less rounded. In T. insigne the sides are always con- 
siderably concave, and the angles hemispherical, a very 
striking character. Distance between the angles •0050''. 
Triceratium quadr angular n. sp., Grev. — Large; valve 
with 4 rounded, somewhat produced angles, and sides with a 
concavity in the middle ; cellulation conspicuous, irregularly 
hexagonal, radiating from the centre ; the cellules becoming 
suddenly very small within the angles, which exhibit no 
decided pseudo-nodule. (Fig. 26.) 
Hab. Barbadoes deposit, Cambridge estate ; C. Johnson, 
Esq. ; very rare. 
A very fine species with a graceful outline, and pale, deli- 
cate reticulation. Margin thickened, or perhaps, rather 
involute, especially at the lateral concavities. The valve might 
be almost described as four-lobed, each lobe broadly elliptical- 
ovate, with the angle produced. Distance between the angles 
•0038". 
Triceratium Atom^us, n. sp., Grev. — Very minute ; valve 
with slightly concave sides and rounded angles, where there 
is the appearance of a small pseudo-nodule across the extreme 
apex; margin with 4 — 5 minute puncta; central structure 
obscure. (Fig. 22.) 
Heb. Barbadoes deposit, Cambridge estate; in slides 
communicated by C. Johnson, Esq.; extremely rare. 
I can make nothing more out of this very minute diatom 
than what is contained in the specific character. 
