66 
Greville, on New Diatoms. 
Heterodictyon^ nov. gen., Grev. 
Frustules free, disciform; disc with radiate or scattered 
cellules or puncta in the middle portion, and a circle of large 
intra-marginal cellules. 
This genus is allied on the one hand to Coscinodiscus, on 
the other to Brightwellia. From the former it differs in the 
circle of very large cellules, from the latter in the absence 
of the spiral arrangement of the central cellulation. Besides 
this distinction, the circle of large cellules seems to be more 
associated with the marginal structure than in Brightwellia, 
at least such is very decidedly the case in one of our species. 
In the other there is an approach towards the last-mentioned 
genus, the circle being further removed from the margin. 
Perhaps the best character will be found to consist (taken in 
conjunction with the circle of large cellules) in the absence 
of the beautiful curved or spiral cellulation which marks the 
three known species of Brightwellia. 
Heterodictyon Rylandsianum, n. sp., Grev. — Disc with 
minute radiating puncta, and a circle of very large, linear- 
oblong, marginal cellules. Diameter '0050''. (Fig. 6.) 
Hah. Barbadoes deposit, from Cambridge estate, in a slide 
communicated by C. Johnson, Esq. 
An exquisitely beautiful disc, and so well marked as to 
require no extended description. Viewed apart from the 
circle of large cellules which occupy nearly a fifth part of the 
radius, there is no character to distinguish it from Coscino^ 
discus, and the resemblance to various species of that genus 
is rendered more striking by the presence of a little central 
cluster of cellules considerably larger than the puncta which 
radiate from them. The large cellules of the margin are 
parallel with each other, somewhat arched at their inner ex- 
tremity, and arranged in groups of three, the middle one 
being the longest. A narrow band of puncta is situated 
between the base of these cellules and the margin. 
Heterodictyon splendidum, n. sp., Grev. — Disc small, the 
central portion occupied with remotely scattered, large, round 
cellules, and surrounded with a circle of large hexagonal 
cellules, having an exterior border of coarse, moniliform strise. 
Diameter -0023". (Fig. 7.) 
Hab. Barbadoes deposit, from Cambridge estate; C. John- 
son, Esq. 
A remarkable, ornamental species, and illustrative of the 
generic name, as no fewer than three conspicuous structures 
