O’Mrara—Report on the Irish Diatomacee. 243 
The reproductive process has not been observed in more than about 
sixty-five species, and in some of these cases there is a difference of 
opinion as to the facts, A system, therefore, in which this process 
constitutes an important part of the ground plan, is practically ob- 
jectionable, as founded on hypothesis. 
The system which appears to me to have most to recommend it is 
that which has been matured by Heiberg, founded on the symmetri- 
cal or unsymmetrical structure of the frustules in their various aspects. 
There are two principal aspects in which a diatomaceous frustule may 
be regarded—the front view, in which the hoop or connecting band is 
presented to the eye, and the side view, in which one or other of the 
two valves is under observation; and in both these positions the 
longitudinal and transverse axes are to be considered. If in these two 
positions, and in these varied views, exact symmetry obtains, the frus- 
tule is said to be symmetrical in all its aspects; but if the two oppo- 
site valves are not uniform, or the portions of the valves on either 
side of the transverse or longitudinal axis, on side view or front view, do 
not exhibit the same proportions or outline, the frustule is said to be 
unsymmetrical on that view or axis on which the difference of form is 
observable. Such is the ground plan of Heiberg’s systematic arrange- 
ment, and which I have adopted in the present report. It is not, 
indeed, wholly free from the objections to which other systems are 
lable, and, possibly, may be open to others peculiarly its own; but 
still the principle on which the arrangement is based commends itself 
as being at once most simple, most comprehensive, and most easily 
applied. 
There are, however, two very important yet subordinate features 
of Heiberg’s system, in regard to which I cannot adopt the views of 
that distinguished author. He ranges the numerous Cuneate species 
as aberrant varieties of the families which in other respects they most 
closely resemble; for example, Meridion and Asterionella are asso- 
ciated with the Fragilariee, under the distinctive appellation of 
Fragilarieze cuneatee ; Podosphenia with the Striatillez, as Striatillese 
cuneate ; Gomphonema and Cocconeis with the Navyicules, as Navi- 
culeze cuneatz ; whereas I have collected the numerous species with a 
cuneate outline, under the one general group of Cuneate. Again, 
the numerous species of Diatomacez, as is well known, exhibit various 
normal phases of growth. Some species are normally free, others 
attached by a short gelatinous cushion, or a larger or shorter stipes; 
the frustules in some genera are simple, while in others, after self- 
division, they remain in concatenate or ribbon-like filaments; in 
some genera the frustules are naked, while in others they are en- 
veloped in mucous fronds, of which some are indefinite, others definite, 
forming simple or composite tubes. These peculiarities of growth 
Heiberg treats as of little significance; and accordingly, the genera 
Schizonema, Berkleya, and Colletonema, the frustules of which are 
included in tubes; Dickiea, the frustules of which are imbedded in 
a less definite mucous mass; Diadesmis, the frustules of which are 
