44 
Wallich^ on Siliceous Organisms. 
supplementary nodules. Front view as in last species^ with 
the last exception. 
Var. with two minute nodules_, in juxtaposition, half- 
way between central and terminal nodules of the median line. 
Length -0038 ; breadth -0007. Figs. 7 and 8. 
Habitat with the last. 
ASTER0MPHALUS_, Ehv. 
The next forms to be noticed belong to the genera 
Asteromphalus and Asterolampra of Ehrenberg. Before 
describing them, however, it is necessary that I should state 
the grounds upon which I have ventured to revise the 
characters of these two genera, and to ignore the supple- 
mentary genus Spatangidium, more recently established by 
M. Brebisson. It is not without considerable hesitation 
that I attempt to modify the classification offered by Dr. 
Greville in the ^ Transactions^ of the Society, only a few 
months ago {' Trans. Microscop. Soc.,^ vol. vii, p. 157). That 
hesitation is, nevertheless, in some degree lessened by the 
knowledge that Dr. Greville^s views have undergone an 
alteration since the appearance of the paper referred to ; and 
by the hope that the opportunities I have had of examining 
a very large number of recent specimens of both genera, may 
obtain for me some claim to be heard. 
The characters of Asteromphalus are — ^^Frustules single, 
equally bivalve, circular ; valves marked with alternate rays, 
forming a double star; central rays (imperfect septa) not 
reaching the margin, two of them parallel, the others 
diverging; marginal rays broader, smooth, flat, one being 
absent or so far obsolete, that the two central rays inclosing it 
become parallel — 
Whilst in Asterolampra the characters are — Free ; frus- 
tules single, equally bivalve, circular ; central portion imper- 
fectly divided by thin septa, which do not reach the margin, 
but alternate with rays extending to the margin, unsupported 
by septa'' {' Microg. Diet.') 
M. Brebisson's sub-genus Spatangidium was devised in 
order to separate such forms as have the rays arranged 
excentrically from those in which they are arranged centri- 
cally, the " secondary or umbilical" rays being very properly 
ignored. 
Dr. Greville, again, proposes to modify M. Brebisson's 
characters by placing, under Asteromphalus, such forms as 
have their discs ^' finely granulated," and a " centrical or ex- 
centrical hyaline area ;" whilst Spatangidium includes the forms 
