DS DRF eee oa 
132 THE MICROSCOPE. 
THE AFFINITIES OF RAPHIDODISCUS. 
C. M. VORCE, F. R. M. S. 
[PLATE VI. | 
OME three years ago Mr. Thomas Christian, of Richmond, Va., 
sent me a small photo-print of what he denominated “the 
missing link” among diatoms. I regarded it as merely a unique 
find of two valves of different families, so superposed and so har- 
moniously constituted in structure as to appear like a single valve; 
and so wrote Mr. Christian. This was the first time I had seen a 
figure of the diatom, which was recognized as the same diatom pre- 
viously described (but not figured, so far as I know), under the name 
of Navicula disciformis, I think, by Mr. Peticolas, of Richmond. 
Later on I saw prints of this diatom under the name of Melon- 
avicula Marylandica; and, finally, in the Microscope for March, 
1887 (Vol. VII, page 65), the generic description of the same diatom 
was published by Mr. Christian under the name of Raphidodiscus, 
accompanied by figures and descriptions of three species. Since my 
first knowledge of this diatom I have hoped to see some further 
consideration of this much-named diatom and its relations, but, 
having no specimens nor time to devote to it, could not undertake 
the task myself. The subject has recently been brought to mind 
afresh by the receipt of one of the boxes of the Postal Mic. Club 
containing a slide, contributed by Mr. Christian, of the deposit, in 
which alone, so far, this diatom has been found; and, having procured 
and examined specimens of the described species, I have decided to 
attempt the unraveling of the confusion in which this diatom is now 
involved. The figures of Raphidodiscus are herewith reprinted 
from Mr. Christian’s article. (Plate VI.) 
In the published characters of the genus (loc. cit.), although 
the generic name is founded on the presence of a raphe, no mention 
is made of its presence as a generic character, which are stated to 
be, in brief, a disciform diatom with cellulate marginal rim, and 
naviculoid centre with interrupted meridian line, and radiate monili- 
form striz prolonged to the margin of the disk. In the description 
of many diatoms the term “line,” or ‘meridian line,” is used in a 
sense quite distinct from that of a raphe or cleft, and it seems to me 
the published characteristics of the genus Raphidodiscus need 
revision and limitation. In one of his letters to me concerning this 
diatom, Mr. Christian referred to it as “upsetting all former 
theories” of the structure and relations of the Diatomaces; in 
response to which I called his attention to the close approach to this 
new form of a number of diatoms of other genera, notably some 
