( = Rhabdonema minutum, Kütz.), they are shortest near the angles, and
gradually longer as they approach the middle. In the latter the
outlines of two plates are frequently seen at one time when viewed
laterally, in consequence of the unequal size of the plates” (see
Plate X X X V III. 306 a & a", Plate XX XIX. 307 b).
We shall find that the septa, here so accurately described, are not
invariably projected equally from all portions of the annuli, but that
an annulus will project a septum from one side only, and the next
annulus a septum from the side opposite its neighbour ; in such cases
the septa appear alternately on opposite margins of the filament, as
■ may be seen in Rhabdonema minutum, Striatella unipunctata and Tabellaria
flocculosa, while opposite septa, where the development takes
place symmetrically around the interior of each annulus, may be noted
in Rhabdonema arcuatum and Grammatophora marina. The analyses
given in Plate X X X V III. 305t, Plate X L II. 314t, Plate X L III. 316t
& 3 l 7t j will enable the student to understand the above description,
and will very clearly exhibit to the eye the singular structure I have
attempted to describe.
I t will be seen that the annuli in the present genus are furnished
towards the circumference with a single series of oblong cellules;
these give a striated appearance to the F. V. of the frustule, and the
number of these striæ wiH accurately represent the number of annuli.
I t is hardly necessary to say, that the peculiar structure of the frustule
separates the present genus from any previously described, the
form of its filament from Tabellaria and Tetraeyclus, the coherence
of its frustules from Grammatophora, and its cellulate annuli from all
these and Striatella. I have detected conjugation and the formation
of sporangia in one of our native species, viz. R, arcuatum. R. minutum,
though occurring in great abundance in various localities and
seasons, on the coast of Sussex, has never presented this phænomenon
to my observation.
1. Rhabdonema arcuatum, Kiitz. Filament slightly arcuate, sub-
stipitate ; annuli from 9 to 30, 6 in •001" ; septa entire, opposite,
slightly and variably unequal in length ; valve linear-elliptical ;
striæ not extending to the extremities of valve, 16 in •OOl". Width
of filament -0011" to -0041". Breadth of valve -0006". v.v.
Kütz. Bacill. xviii. 6. Diatoma arcuatum, Lyng. Tent. Ixii. B. Conferva
striatula, Eng. Bot. 1928. Diatoma striatulum, Grev. B. F. p. 405.
Striatella arcuata, Ag. Consp. p. 61. Ehr. Inf. xx. 6. Ilarv. Man.
p. 199. Ralfs, Ann. vol. 11. pi. ix. 6. Prit. Anim. iv. 203, 204.
Marine. Hastings, Oct. 1851. Cuckmere, Sussex, April 1852. Poole
Bay, Sept. 1854, W. Sm. Penzance, Mr. Ralfs. Coast of Sussex, Mr. Jenner.
Frith of Clyde, Feb. 1854, Dr. Arnott.
Plate XXXVIII. 305 f 305 a, & 305t.
2 . Rhabdonema Adriaticum, Kiitz. Filament arcuate, substipitate;
annuli from 15 to 60, 8 in •OOl"; septa perforate, alternate,
gradually decreasing in length as the annuli recede from the valves ;
valve linear-elliptical; striæ not extending to the extremities of
valve, 24 in -OOl". Width of filament •0011" to -0054". Breadth
of valve -0004" to -0006". v.v.
Kütz. Bacill. xviii. 7.
Marine. Poole Bay, Nov. 1849, Sept. 1854, and Cork Harbour, Oct. 1855,
W. Sm. Belfast Bay near Carrickfergus, July 1854, Mr. Hennedy. Frith of
Clyde, Feb. 1854, Dr. Arnott. Ilfracombe, Miss Hodgson. (New York and
Mauritius ; communicated by Dr. Arnott. Mediterranean Sea near Marseilles,
May 1854, W. Sm. Near Jaffa, Mr. Johnson.)
The above species is much less frequent than the former on the British
coast, and I was unable at first to satisfy myself of its distinctness. Tbe
specimens which I collected in the Mediterranean, as well as those supplied
by Dr. Arnott from New York, have enabled me to examine the form more
thoroughly, and I am now satisfied that it cannot be regarded as a mere
variety. The characters I have given are constant in the smallest as well as
the largest filaments, and the relative length and alternate arrangement of
the perforate septa are so obvious in prepared specimens, that there can be no
difficulty in the determination of the species.
With the present, which occurs very sparingly in the gathering from Mauritius,
there is another magnificent species with filaments occasionally reaching
•0086" in width, and with alternate and cribrose septa ; the latter give a very
brilliant appearance to the aspect of the frustule. For this species I propose
the name of R. mirifieum. I have the same in a gathering from Ceylon, supplied
by Dr. Kelaart.
All the species of the present genus occasionally suffer a partial displacement
of their frustules, which afterwards continue to cohere by their angles ;
but this partial rupture is evidently due to external violence, and never occurs
with that regularity which gives to Diatoma, Tabellaria and others the
notable zigzag which characterizes their filaments.
Plate XXXVIII. 305 b. & 305 a' & V.
3. Rhabdouema minutum, Kiitz. Filament direct or siightly flexu-
D 2