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daitoms are quite free, and arranged uniformly in transparent tubes 
of considerable density and firmness (.Schzzonema), while in others 
they are clustered in masses, according to a greater or less definite 
plan, the investing substance being less consistent, and partaking 
more of the character of mucus than the tubes of the former 
(Mastagloia). 
Specimens of the latter I have found about half-way up the 
cliff above the sewage-outlet, which is indeed the only place in 
which I have ever found them. The former—viz. those in gela- 
tinous tubes—may be had in many varieties and great quantities 
in the rock-pools around Tom Hurd’s Rock, below Wellington 
Pit. Their presence may be known by the velvety appearance 
they give to submerged stones on which they grow. 
To the microscopist and the optician, the lines on the valves of 
some genera of the Diatomacez are especially interesting ; which, 
because of their great number and extreme delicacy, are much 
esteemed as test-objects for ascertaining the defining quality of 
microscope object-glasses of high magnifying power. Of these 
markings, or strize, a great difference of number, distribution, and 
development is found to exist among the several genera. Not 
unfrequently, two or more sorts of markings are present in the 
same individual, and onthe same valve (Heliopelta, Actinoptychus). 
The entire surface may be covered with certain bands or dots, or 
both, as is the case with all the members of the first great section 
(Wavicule), which has already been named as having a longitudinal 
line and central nodule. The number of these lines, or striz, 
present on the frustules of some of the species of Pleurosigma 
and JVavicula are, if possible, more incomprehensible and con- 
founding than the expressed magnitudes and distances of the 
heavenly bodies. For instance, within a linear inch of space, 
clearly and sharply defined, P. formosum has 34,000, £. balticum 
has 38,000, P. hippocampus has 40,000, P. guadratum has 43,000, 
P. angulatum has 52,000, P. fasciola has 52,000, and Navicula 
rhomboides has 100,000 such lines. 
The mode of increase of the Diatomacez is twofold. The 
most common, and what was long thought to have been the only 
