different aspects of tlie frustule. It would have been desirable
to have here adopted the terms employed by former writers,
especially by those distinguished authors who have led the
way in the study of these organisms; but this course is precluded
by the circumstance that these writers have employed
terms which imply views of the structure and nature of the
Diatomaceous frustule that are now altogether inadmissible, or
at all events at variance with the conclusions of the present
writer.
Thus, the terms ventral and dorsal, employed by Ehrenberg,
would be clearly inconsistent, if not unmeaning, when applied
to the different aspects, or parts of aplani. The terms primary
and secondary sides, adopted by Kiitzing and others, are not
open to the same difficulty ; but they labour under this objection,
that they have not been employed by Mr. Ralfs, the highest
English authority upon the subject. I shall therefore adopt the
nomenclatm-e of the last-named writer, as the most convenient
for the English student, and use the term “ front view,” to denote
the aspect of the frustule when the valvular suture, or the
line along which self-division takes place, is turned towards the
observer ; and the term “ side view,” when the centre of one
valve is directed to the eye.
Even these terms will require modification when applied to
some of the more complex and irregular forms ; but in general
their meaning will be sufficiently obvious, and special cases wül
be noticed as they present themselves.
Self-division also supplies circumstances and distinctions,
which appear to me most suitable in the present state of our
knowledge, on which to found a generic arrangement of the
Diatomaceæ. The circumstances which accompany the Reproduction
of these organisms are so imperfectly ascertained, and
that in so few species, that it is impossible to employ them with
advantage in a generic arrangement. Self-division seems to me
to come next in order, as a most important function connected
with increase and growth, and to supply the necessary variety
of phænomena on which to ground our sectional divisions.
I have therefore separated those forms where self-division is
accompanied by the secretion of a permanent gelatinous or
membranaceous envelope, in which the frustules arc subsequently
imbedded, from those in which such secretion is altogether
absent, or is represented merely by a cushion or stipes, to which
the frustules arc attached by a small portion of their surface ;
and I have placed the latter, as of simpler organization, in my
first tribe, arranging the genera belonging to it into sub-tribes,
depending upon the permanency or otherwise of the connecting-
membrane, another product of the self-dividing process. This
enables me to place apart those genera whose species present us
with frustules in which the union of the cells is dissolved almost
immediately upon the completion of self-division, as well as those
where a cushion or stipes still maintains a kind of indirect individuality
in the divided frustules, from the genera in which
the cells cohere after gemmiparous increase, and by such coherence
form filaments of various lengths and forms, allotting
the latter to sub-tribes which respectively present a compressed
filament, a zigzag chain, or a cylindrical thread. In the second
tribe, including those genera which have frondose forms, I find
characters for my sub-tribes in the nature of the frond and the
arrangement of the frustules.
I do not propose this arrangement as free from exceptions or
even serious defects, but I have adopted it in preference to those
hitherto given, as bringing more frequently together forms
allied in structure and mode of growth, and as being at the
same time more strictly in accordance with the external physiognomies
of these organisms, and therefore more likely to be
apprehended by the inquirer entering upon the study of this
department of nature. A wider study of Diatomaceous forms
will doubtless lead to more accurate and more natural generalizations.
S e c t io n VII.
On C o l l e c t in g a n d P r e s e r v in g S p e c im e n s o p t h e
D ia t o m a c e æ .
I have already described the various localities in which the
Diatomaceæ ordinarily abound. Supposing the observer to be
à .