50 
Wallich_, on Siliceous Organisms. 
tlieir association, in tlie recent state_, with otlier organisms, 
sucli as Diatomacea, Foraminifera, and Acanthometra, whose 
fossil remains, in like manner, occur associated with them in 
the flints. The question as to their being true sporangia of 
Desmidiacem becomes, therefore, of secondary importance. 
On endeavouring to identify, if possible, the Xanthidia of 
the S alp 36 material with some of the described fossil forms, 
I was struck by the marked resemblance the first species, 
figured by Mr. H. H. White (in the ' Trans. Micros. Soc, 
for 1847, vol. i, p. 77), bore, not to my Xanthidia, but to 
Cos ci7io discus Sol. The central portion, as rendered in the 
figure, certainly exhibited no trace of hexagonal cellulation, 
such as is to be found in all the Coscinodisci. But the 
peculiar membranous expansion, and its ragged border, tallied 
in so precise a manner with certain specimens of C. Sol 
that had become partially dried, by accident, on the slides 
containing them, that, on turning to the letter-press, it was 
with no small pleasure the following passages struck my 
eye. 
X. vestitum, so named from the transparent membrane 
which extends beyond the body^^ and again, "It has the 
appearance rather of a superficies or disc, than of a solid 
body.^^ 
All doubt on the question has been set at rest, however, 
inasmuch as, through the politeness of Mr. White, I have 
had an opportunity of inspecting the identical specimen from 
which the figure referred to was drawn ; and, not only has 
my examination confirmed the first impression, but lent strong 
additional testimony in its favour, the membranous expansion 
of the fossil specimen proving to be unquestionably double, 
as is the case with the recent one. 
In sections of flint in my possession, the remains of the 
obliquely truncate cylinders of Rhizoselenia and spicula of 
of Acanthometrae may be distinctly recognised. Both these 
forms are also plentifully found, in a recent state, in the 
Salpse stomachs. But, as in the case of the fossil disc in Mr. 
Whitens collection, all trace of markings on the siliceous 
envelopes are lost. The soft contents have retained the 
original outline, and the geometrically arranged spicula of 
Acanthometra are represented by the dark metamorphosed 
granules of sarcode, which, in the living condition of the 
organism, had occupied its tubular cavities and nucleus. 
In the recent state the siliceous organisms had been asso- 
ciated with the Xanthidia. In the fossil state, they still 
were found together. But whence had vanished the beautiful 
and very conspicuous network of the disc ? It had yielded 
