Wallicii, on Siliceous Organisms. 
39 
markings^ with the exception of the lines referred to. These 
take their rise from a row of minute marginal puncta^ and, 
in like manner with the latter, vary in number in different 
specimens. Where they emerge from the disc, they present 
the appearance of being puckered or folded, becoming gra- 
dually linear as they approach the outer margin of the ring. 
In specimens preserved in their original state in fluid, both 
valves may be seen to possess the ring. It is evidently, there- 
fore, an emanation from the valvular disc, and not from the 
connecting zones ; and it may be regarded as a modification 
of that portion of the primordial utricle which invests the 
minute marginal apertures of the valve, and which, in the 
filamentous, stipitate, and sessile forms, assumes the nature 
either of an elastic cushion, a stipes, or a pedicle. 
Of the highly elastic character of these extra-frustular pro- 
ductions we find abundant evidence in the tenacity with which 
the filamentous species cohere by one or both their angles, 
as in Himantidium, Fragillaria, and Biatoma; in the manner 
in which the long-stipitate species sustain their heavy burden, 
as in Acnanthes and Striatella ; in the modification of the 
same structure observed in the highly elastic tubular sheath 
of Schizonema ; in the matrix of the frondose forms, as in 
Mastogloia, Dickieia, and Berkleyia ; in the nidus of all spo- 
rangial frustules or cells ; and still more remarkably, as I 
conceive, in the delicate enveloping medium of Bacillaria. 
On referring to the description given in the ^ Synopsis of 
British Diatomacese,^ vol. ii, p. 9, of the movements observed 
in Bacillaria paradoxa, the author remarks that if the fila- 
ment, while in motion, be forcibly divided, the uninjured 
frustules of each portion continue to move as before, proving 
that the filament is a compound structure, notwithstanding 
that its frustules move in unison.^^ Now it is difficult to 
reconcile the view of a compound structure, as here conveyed, 
vrith what we know of the ordinary diatomaceous frustule, 
unless by admitting the presence of a highly elastic and deli- 
cate investing membrane, such as I contend for. In the 
usual acceptation of the term, all filamentous forms are com- 
pound ; and yet, in no other diatom do we discover the very 
remarkable combination of movements witnessed in B. para- 
dox a. 
I have tried to prove, in the paper already spoken of, that 
endosmotic and exosmotic action cannot possibly account either 
for the motions seen in B. paradox a, or indeed in any other 
diatom with which we are acquainted ; and that, in order to 
explain the ordinary movements, and also those very striking 
secondary movements exhibited by objects in the immediate 
