I l f
2. A ch na nth e s e x il i s ? Kutz.
Plate C. Fig. 5.
Char. Frustules slender, from one to nine in each frond.
Lateral surfaces subacute. Striæ indistinct or wanting.
Sti[)es longer than the fi'ustide.
Kiitz. Syn. Diatom, p. 578. f. 53. ; Ehr. Infus. p. 228.
pl. XX f. 5. ; Ealfs, in Annals, vol. xiii. pl. xiv. fig. 12.
Hah. Parasitic on Gomphonema gerninatmn in several
streams, near Dolgelly : Air. Ralfs.
From Achnanthes minutissima this species differs in its
elongated stipes, greater number of frustules in each frond,
and in its more acute lateral surfaces.
Although I have compared our plant with Kutzing’s
Achnanthes exilis I am not certain th a t it is the same species.
I t agrees with Kutzing’s specimen in its crowded habit and
elongated stipes, but its frustules ai'e much smaller, and
its lateral surfaces less acute, in both which respects it is intermediate
between Ach. minutissima and Kutzing’s specimen of
Ach. exilis.
B. Filaments compressed.
A. Frustules in fro n t view quadrangular^.
68. T E T EA C Y C L Ü S feaZfs.
Char. Frustules cohering so as to form a fragile, attenuated
filament, never united hy their angles, striated strongly
laterally ; four-sided, each side rounded, and forming the
segment o f a distinct circle.
Derivation. From Tsrpas, fo u r, and k v k K o s , a circle.
This genus differs'only from Tessella and Sti'iatella in the
frustules not cohering by the angles.
1. T etkacyclus lacustris Ralfs.
Plate X C I I I . Figs. 8, 9.
Char. Frustules about twice as long as broad. Lateral surfaces
with from seven to nine distinct transverse striæ ; in
end view four-lohed.
^ Hab. L lyn Prefeddyr, near Barmouth ; Rev. T. Salweg.
Pools near Dolgelly : Mr. Ralfs.
In consequence of the four-lobed form of the frustules
composing a filament, and which arises from the inflation of
the central part of each frustule, it happens that when examined
by the microscope that the centre of one side or lobe
is seen together with a portion of two other lobes, one on each
side, the boundaries of the lobe in view being indicated by two
lines running down the length of the filament. But as the
lobes are npt all of equal size, those only being so which are
opposite to each other in the filament, it follows tha t the
quantity of the other two lobes seen varies according to the
side of the filament which is uppermost. Now, the longer sides
or lobes are those which form the margins of the filaments, and
those which form the centre the shorter, the filament rests of
course usually upon one of the shorter lobes ; from this it results
that in the filaments in their usual position, that a considerable
portion of the lateral lobes is seen, but in the cases in
which they rest upon their longer or marginal lobes a very
small portion only of the lateral lobes is in view. The smaller
and shorter lobes correspond to the back and front of the filament,
and the larger and longer to the margins, which may be
distinguished in addition to their breadth by the presence of
striæ, more strongly marked than they are in the centre of
the filament : the terminations of the frustules in this view
are also punctated, although not contracted. In the marginal
view, tha t is with the margin of the filament uppermost, but
a very slight portion of the back and front lobes is seen, the
striæ are most strongly marked in the centre, and describe
quadrangular spaces, the frustules are contracted also at their
points of junction where they present no puncta.
D D 2