57 
diatoms, like those larger plants, seem ubiquitous ; they are found 
existing in equal plenty amid the snows of the Arctic regions, 
the heat of the Tropics, and the pancake ice of the Arctic and 
Antarctic seas. Others are limited to very small areas ; for ex- 
ample, we have living in Ormesby Broad a small species, which 
has never been found in any other locality ; another species 
occumng in a fossil state in Franzenbad, in Bohemia, has been 
found in great abundance in a ditch near the Berney Arms, and 
so far as I am aware, nowhere else. Some genera are only found, 
in any quantity, on tlie sand ripples left by the retreating tide ; 
others form part of the surface flora of mid-ocean, and appear to 
be the food of the salpte and noctilucm ; and the student availing 
himself of the stomach contents of those organisms, adds many 
rare forms to his collection. The salpas, noctilucae, &c., are eaten 
by fish — these again form the food of the various species of marine 
birds, the producers of the enormous dej)osits of guano. These 
deposits yield up their diatomaceous riches to the careful manipu- 
lator, and our little diatom, after undergoing such strange vicissi- 
tudes, is at last entombed in Canada balsam, and placed among 
the choice slides of the microscopist. 
The diatomaceous frustule may be compared to an ordinary pill-box, 
in which the bottom has been replaced by a lid ; the two lids are 
termed valves — the body of tlie box the connecting zone or 
cingulum — the surface of the valve is called the side view — that 
of the cingulum, the front view. In an early stage of growth the 
cingulum is very narrow, and, I may here remark, that this is the 
only part of the frustule that really grows ; the valves never 
increase, but, on the contrary, decrease in diameter as reproduction 
proceeds. The cingulum increases in breadth only, and being of 
later formation, is less firmly siliceous than the valves ; in its 
earliest stage of formation the frustule is probably little more than 
a membranous sac, and when this membrane is exposed to the 
action of water, the secreting process at once commences. The 
cell membrane upon which the silex is deposited, is perhaps the 
analogue of tlie primordial utricle of the ordinary vegetable cell. 
That such a membrane exists, has been conclusively proved by an 
experiment instituted by Professor Bailey ; he subjected the 
frustules of recent diatoms to the action of hydrofluoric acid, and 
dissolved away the silex, leaving an elastic membrane unattacked 
