Gbeville, on New Diatoms. 
19 
No correct representation of this fine species having hitherto 
been published^ I have been induced to figure a frustule of 
extraordinary dimensions found by Dr. Macrae in Ceylon. 
When Professor Smith wrote the first volume of his ^Synopsis 
of the British Diatomacese/ it was considered so rare that 
some of the slides distributed by him contained only a 
solitary carefully marked specimen. The figure engraved in 
the synopsis is incorrect in outline and deficient in details. 
Doubtless, however, examples must have occurred to the 
author of an ovate form, and such a one must have been 
placed before the engraver ; but Mr. Ralfs remarks Pritch. 
Inf.^), we have never seen it ovate as described by Professor 
Smith and I may add my own testimony to the same eff'ect,, 
after having examined a multitude of frustules. The ordinary 
form is unquestionably elliptic, with rounded ends, as stated 
by Ralfs, occasionally tending to orbicular, as in the specimen 
now figured. An error seems also to have been entertained 
with regard to the structure of the centre of the disc. Smith 
does not refer to any striation whatever of what he calls the 
median line, and the figure in the synopsis has the centre 
wholly blank. On the other hand, the median space is de- 
scribed in ^ Pritchard's Infusoria ' as transversely striated, a 
character I have been unable to perceive. After making the 
drawing of the gigantic individual received from Dr. Macrae, 
I examined an extensive series of frustules from British 
stations, relative to the structure of this part, as well as a 
slide from Professor Smith himself, and find them all to agree 
in essential points. It will be seen, by consulting my figure, 
that the median space is enclosed by a narrow line of exceed- 
ingly short striae, within which the costse are continued 
towards a sort of indefinite median line, indicated by the ter- 
mination of the costse rather than by any genuine line. The 
normal shape of the median space is correctly described by 
Mr. Ralfs as lanceolate ; but it varies exceedingly, and I have 
seen frustules in the same slide exhibiting every gradation 
from a strictly linear to a broadly lanceolate median space. 
In the former case the narrow, striated lines become parallel^ 
and no room is found for any continuation of the costae. In 
no instance, however, so far as I have seen, do the strise really 
cross the median space. In proportion as the striated lines 
expand, continuations of the costse are introduced, at first 
with great irregularity, being short, often pointing in different 
directions, frequently passing across the middle, and scarcely 
appearing to be connected with the costse at all. As the 
median space increases, they become more regular, and in 
largely developed frustules^, as in the figure now given^ there 
