42 
Wallich_, on Siliceous Orgmiisjns. 
whatever. It is unnecessary to touch Qn the rhythmical alter- 
nating nature of the movements of the Diatomacese ; we have 
many examples of analogous action^ in both the animal and 
vegetable kingdoms. For^ whether we examine the heart's 
action of one of the most highly organized animals^ or the 
contractile vesicle seen in the animalcule^ and even in some 
forms of the early vegetable cell^ we are met by the same 
wondrous fact_, and perplexed by the same insurmountable 
difficulty. 
Hemidiscus^ nov. gen._, Wall. 
Frustules free. Valve arcuate^ with a marginal nodule. 
Cellulation hexagonal^ radiate. 
The valvular structure of this genus indicates a very close 
affinity to Eupodiscus, and the discoidal forms generally. 
Its peculiar outline,, and the invariable character of the 
unequally developed connecting zones^ distinguish it from 
that genus. We have examples of similar unequal develop- 
ment in the connecting zones of some Surirellce, Gomphonema, 
Podosphenia, Rhipidophora, Licinophora and Meridion ; in all 
of which genera the cuneate^^ character of the frustule is, 
in a great measure, due to its occurrence. 
H. cuneiformis, n. sp. — Valves arcuate, with a slight in- 
flation and nodule at the centre of the plane margin ; and a 
row of marginal puncta. Valve slightly convex at its margin. 
Cellulation distinct ; hexagonal ; largest at centre, from 
whence the cells radiate. Connecting zones broadest on the 
convex aspect of the frustule, the section of which is, accord- 
ingly, cuneate. Length -0017 to '0056. Diameter of valve 
•0010 to -0025. PI. II, figs. 3 and 4. 
From Salpse, Bay of Bengal and Indian Ocean. April, 
1857. 
This species presents gTcat variation in size. In general 
shape the frustule corresponds with that of the Brazil nut of 
commerce ; the flat sides representing the valvular surfaces, 
and the broad convex back the broad portion of the connect- 
ing zones. Sometimes, however, the valve attains a much 
more arched character, and then appears almost semicircular. 
I have repeatedly observed detached valves of this form in 
several of the guanos. As seen in these, the valves might 
readily be mistaken for abnormal varieties of a Triceratium ; 
and it is just possible, therefore, that the Triceratium semi- 
circulare of the Bermuda deposit, described and figured by 
Mr. Brightwell in the ^ Microscopical Trans.' (vol. i, p. 256) 
may belong to this genus. In the figure appended to Mr. 
BrightwelFs paper, the arrangement of the cells seems to 
