slighter variations in each species, sufficiently constant to form
a safe guide to their- determination, is the influence which the
presence of the siliceous element, in the composition of the valve,
has, in modifying the cellulate character of its structure, and
producing diversities in the number, extent, and form of its
markings.
These variations of structure arise from the modes in which
the silica combines with the cellulose of the epiderm ; and this
combination seems to obey certain and invariable laws, which
are subject to but slight derangement from the external circumstances
in which the growth of the embryo takes place. The
more important variations arising from this cause exhibit themselves
in the presence or absence of aggregations of the siliceous
material in the form of a median rib or line with its central and
terminal nodules, under several very characteristic modiflcations
—in the presence or absence of other transverse and strengthening
bands of silex, and in the obvious varieties of form or combination
to which the cellules submit in the progress of their
formation, exhibiting themselves as hexagonal, circular, or irregular
in outline, as distinct from each other, or as more or less
confluent. These characters, in some cases useful as specific
distinctions, are more frequently available in the arrangement of
the genera and their subdivisions. Less conspicuous features in
the cellulate structure of the valve will be found necessary in the
determination of specific forms.
Whether the striæ, resulting from the cellulate character of
the structure, are mutually parallel or radiate in their arrangement,
reach the median line, or are absent from a greater or
lesser portion of the valvular surface—whether the cellules
themselves are arranged in squares or disposed in quincunx,
and the striæ in their transverse direction are consequently
parallel or oblique in reference to the margin or the median line
—the relative distances of the striæ and their greater or less
distinctness,—all these are features which may safely be regarded
as of specific importance, and though subject to slight modifications,
arising from the accidents of locality and age, are sufficiently
constant to enable us to associate together frustules which owe
their birth to the same sporangium, and which, on a renewal
of the conjugating process, will produce frustules with the same
specific characters as themselves. In describing a species, we
should therefore carefully note the character of the striation, and
state as nearly as possible the average distance of the striæ.
The dry valve will frequently aid in such determination, the
presence and extent of the striation being usually indicated by
the colour of the surface, and the differences in colour not un-
frequently answering to the relative distances or distinctness of
the striæ.
Another feature, which appears in its modifications to confer
a specific character upon the Diatomaceous organism, is the
arrangement of the endochrome or cell-contents in the living
frustule. These modifications are familiar to those who are
accustomed to the examination of freshly gathered specimens of
the Diatomaceæ, and are found to be constant and characteristic
of each specific form. In one case the endochrome is closely
applied to the inner surface of the valve ; in another, aggregated
in the centre of the frustule; sometimes sparingly diffused
throughout the interior, or again exhibiting a radiate or stellate
arrangement; at all times having one or several oily(?) globules
which occupy in different species different positions, but are
constant in number and situation in the same species.
Should both these methods of determining the species fail the
inquirer, there remains a third which will often aid him in the
desired discrimination, viz. the locality in which the specimen
has been discovered. I feel persuaded that a marine species
will not flourish under fluviatile influences, nor a freshwater
form long survive when transferred to a marine habitat. Still
further, I believe that certain species are far more special in
their tastes, some selecting mountain-torrents, others clear and
still waters ; some preferring the deltas of rivers, and others
fixing their habitation in boggy pools or alpine lakes; some
being exclusively littoral, and others found only in the deeper
parts of the sea.
Where structural diflferences are not obvious, when striation
is indistinct or too minute for detection, and the arrangement
of the cell-contents is modified by the death of the frustule, a
knowledge of the habitat of the specimen, when in a living state,