136 THE MICROSCOPE. 
double diatom, and can be photographed so as to show either one 
predominant. Of the four “Christianii” valves on this slide, one 
has its rim broken away for about one-quarter of its circumference, 
and it is perfectly apparent that the adjoining valve of ‘‘ Febigerii” 
differs from it only in having its rim broken away all around the 
valve. 
The genus Raphidodiscus must, therefore, so far as the forms 
at present known are concerned, be limited to a single species, 
R. Christianti, and the two species, Marylandica and Febigerit, 
must be deleted. 
Since the above was written, Dr. Ward has furnished an 
amusing but conclusive proof of the foregoing conclusion regarding 
R. Marylandica, by the very ingenious expedient of placing a valve 
of R. Christianii into a valve of Melosira and mounting the 
combination as a single valve. This he successfully accomplished in 
two cases, and it is found impossible to see the edge of the valve of 
Raphidodiscus in these specimens, although its exact locality is 
known. Photographs of this combination, which Dr. Ward 
facetiously calls ‘“‘Raphidodiscus bogus, N. Sp.,” show it to be 
indistinguishable from ‘‘ Marylandica,” and compare exactly in the 
structure shown with the photographs sent out by Mr. Christian, 
labeled ‘‘ Melonavicula Marylandica.” 
It follows necessarily that the characteristics of the genus 
Raphidodiscus, having been founded on a combination instead of a 
true specimen, must be revised. The cellulate margin must be 
struck from the list, and the raphe and terminal spines must be 
included. I propose the following description of the generic char- 
acteristics of Raphidodiscus, viz: 
“ Rapuripopiscus.— Valves discoid, with a central thickening or 
obscure nodule, and an interrupted raphe terminated by minute 
spines or spiniform nodules somewhat within the margin of the 
disc; central portion of disc naviculoid, depressed, its ends 
terminating at the spines; striz radiate, moniliform, extending 
from raphe to the margin of the valve.” 
As no definite specific description accompanied the figure of 
R. Christianii when published (loc. cit.), I offer the following : 
“R. Curistranu, Gascoigne.—Valve flat, with rounding, slightly 
thickened rim; central nodule obscure, often obsolete; raphe 
distinct, each part about one-third diameter of valve in length ; 
terminal spines minute, and placed about one-sixth diameter of 
valve from margin; naviculoid central space large, depressed, and 
defined by a thickened line or ridge, extending to or slightly beyond 
