Shadbolt on New Forms of Diatomacece. 
15 
Of the genus Triceratium there are no less than five new 
species, one old, and one doubtful, making seven in all. 
The first I shall notice is of moderate size, and is distin- 
guished by peculiarly delicate markings somewhat obscurely 
disposed about three equidistant pseudo-nuclei — Tr. sculptum 
(^^■^^- . . 
Next we have one in which the markings appear like 
minute dots, closely crowded together, but disposed in a very 
regular manner from the axis of the valve ; this is about the 
same size as the preceding, but the outline differs materially, 
each margin being arcuate with the concave surface outwards 
— Tr. arcuatum (fig. 5). 
Another species of medium size is in form nearly the con- 
verse of the preceding, the margin being so inflated as to cause 
the triangular outline to approach that of the circle ; hence 
Tr. orhiculatum as its specific designation (fig. 6). This 
species exhibits a structure similar to that of Coscinodiscus 
radiatus: the reticulations, however, are not so regularly 
hexagonal, but they are largest at the centre, and diminish 
in size gradually towards the margin of the valves. 
1 have noticed also a single specimen of what appears to 
be Tr. alter nans, but, as it presents only its front view, it is 
difficult to determine. 
The next I shall allude to is, however, the most important, 
being very remarkable and especially interesting from the 
front view exhibiting the disposition of the horn-like append- 
ages (figs. 7 a and 7 b) and the mode of dividing. On the 
lateral view of the valves the most striking peculiarity is a 
sort of twistedness in the angles (fig. 7 «), which is very 
marked : the specific name proposed is to note this fact, viz. 
Tr. contortum. 
The surface of each valve is adorned with short spines 
arranged in a tri-radiate double row, and at the termination 
of each double row is one very long one, being about one- 
third of the length of a side of the valve. These long spines 
are independent of and placed nearer to the axis than the 
horn-like processes from which the genus derives its name. 
Fig. 7 h shows a specimen undergoing self-division. 
Another new species, much smaller than the last, is charac- 
terized by the reticulations being coarse and irregular in form, 
and the horns very large as compared with the size of the 
valve — Tr. orassum. 
Another species is, I believe, a variety only of the T.favus, 
which has been called gibbosus. 
The genus Pleurosigma has no less than five species, two 
being quite new, and both having the markings arranged dia- 
