Greville_, on New Diatoms. 
81 
rays^ are somewhat irregular in length, and consequently do 
not form an exact circle. They terminate in one or oc- 
casionally in two spinous processes, which are evidently 
analogous to those with which some of the rays in C. armatus 
and other diatoms are furnished. 
Triceratium. 
The first seven of the following species constitute a very 
interesting and exceedingly natural little group, and present 
an excellent illustration of the difficulty of distinguishing 
between closely allied forms. Without attempting to dog- 
matise upon the questio vexata of " What is a species ? we 
may safely venture to figure and describe, with benefit to 
science, such organisms as we have reason to believe exhibit 
characters by which they may at any time be identified. 
Such characters are necessarily sometimes minute, but are not 
thereby of less value. In a systematic work the species 
about to be described would arrange themselves at once into 
two sections — the first containing those which have simple 
(not striated) margins and the central triangular space 
filled up with radiating lines ; the second those which have 
striated margins and the central triangular space blank. 
There is another peculiarity, also, which separates the two 
sections. In the first the angles of the central triangle are 
lengthened out until they reach the pseudo-nodule ; in the 
second the angles are not lengthened out, but each is kept 
with a short strong line which never reaches the pseudo- 
nodule, but terminates in a fork more or less connected with 
other vein-like lines. I have not satisfied myself about the 
nature of the short line referred to. In T. pulcherrimum 
(fig. 6) it takes the form of a small spine, distinctly seen 
within the pseudo-nodule. In T. marginatum it may also be 
seen, but with some difficulty, through the intervening lower 
pseudo-nodule. These little spines must be regarded as 
analogous to the short lines holding a similar relative position 
to the angles of the inner triangle in the species of the 
second section. In some instances, especially in T. varie- 
ffatum, 1 have observed the short line to be slightly raised, 
suggesting the idea, which is confirmed by the position of the 
spine in the species of the first section, that this organ 
belongs properly to the framework of the inner triangle, and 
that the lines which appear to emanate from it belong to the 
system of costae or vein-like lines which divide the border of 
the valve into compartments. 
Triceratium marginatum, Br. — Valve with slightly convex 
