Wallich, on Siliceous Organisms. 
45 
with " distinctly areolated discs^^ and an excentrical hyaline 
area/^ 
The confusion into which these genera have fallen appears 
to have arisen from the frindamental error of viewing the 
sutures of the basal portion of the true rays as " secondary 
or umbilical rays/^ Omitting, for a moment, the question as 
to the true nature of the " septa/^ or " imperfect partitions/^ 
spoken of by Ehrenberg, and assigned by him both to 
Aster omphalus and Asterolampra, a marked character exists 
by which alone these two genera may be distinguished from 
each other. It consists in the presence, in the first-named 
genus, of that ray variously designated as the median,^ ^ 
^' nuclear,^^ and " obsolete^^ ray ; and for which I propose to 
substitute the name basal ray, from the true rays being 
always arranged around or upon it. But in addition to this 
primary character it will be found, on looking at the front 
view of frustules of the two genera, that an important 
generic character may be derived from the internal structure 
of the valves ; ^' the imperfect partitions'^ being seen to dip 
down towards the interior of the frustule of Asteromphalus , 
whilst they do not occur, at all, in Asterolampra, save as 
linear markings similar to those seen on the external surface. 
Simple repetition of parts, such as the rays of these 
genera, and trifling modifications in outline, must be looked 
upon as insufficient bases for specific distinction. In like 
manner the centrical or excentrical arrangement of the rays 
is a character on which no reliajice can be placed, as both 
may be seen to occur in different individuals indubitably 
belonging to the same species. As a general rule, a certain 
degree of excentricity must exist, as the natural result of the 
presence of a basal ray. Nor could it well be otherwise, 
when we reflect that the diatomaceous frustule, once formed, 
admits of no change of structure, and that the excentricity 
is therefore the result, and not the cause, of the peculiarity 
in the basal ray. That some definite function is performed 
by the rays must be inferred from the fact that, in numerous 
specimens preserved in balsam, some of the rays may be seen 
filled with an air-bubble, proving that they are tubular. But 
so long as the true and basal rays do not deviate in structure 
from their normal types, mere number, and the simple 
amount of excentric arrangement, may safely be regarded as 
depending on causes not of a constant character."^ The rays, 
* lu the October number of tlie 'Quarterly Journal of Micros. Science,' 
Mr. Koper, \Aiiilst commenting upon Mr. Johnston's paper, draws attention 
to the fact of Mr. Shadbolt having pointed out, some years ago, the " alter- 
nate disposition" of the valves in Asterolamp-a, The same structure has 
VOL. viii. g 
