which suiüciently distinguishes this and the following genus ; while
the presence of a stipes of more or less perfect development separates
Achnanthes from its ally. The peculiar form, conspicuous size,
and general diffusion of the several species of the present genus
place them among the most singular and interesting of the Diatomaceæ,
and have attracted the attention of all observers ; still but
very little is known of the circumstances which control their development,
and conjugation has been noticed in one species only,
and that by myself alone. The anomalous form of the valves, and
the exceptional character of their markings, I am wholly unable to
explain; the additional strength given to the lower valves by the
presence of nodules and a transverse band of silex being apparently
uncalled for by any peculiar stress or demand upon this portion of
the frustule.
Some confusion has been introduced into the genus by adopting
the length of the stipes as a specific character. This feature, though
constant in some, varies greatly in other species, and ought not therefore,
m the latter cases, to be regarded as an important distinction ;
Its comparative length or even presence in such species wholly depending
upon the stage of growth at which the filaments have arrived.
Nor is the length of the filament, into which the united frustules
cohere, a satisfactory criterion, the coherence being apparently dependent
upon accidental circumstances, and varying with the habitat
and season. The form and striation of the valve are more certain
and constant characters, and these, though subject to slight modifications,
are our best guides in the discrimination of species. The
habit of growth in the stipitate species will also be found an important
aid, the filaments being sometimes scattered over the larger
Algæ to which they are affixed, sometimes springing from a point
around which their stipes cluster in considerable numbers. I t will
be seen from the figures given, that the connecting membrane in the
present genus is usually delicately striated, a character which has
been before noticed in Himantidium, and will be found in others of
the filamentous Diatomaceæ.
1. Achnanthes longipes, Ag. Filament of few frustules, from 2 to
16; V. linear-elhptical, obtuse, with a slight central constriction,
costate; costæ unequally distant, from 6 to 12 in -OOl"; striæ
moniliform, 27 in -OOl". Length of frustule -0019" to -0072".
Stipes usually clustered, from •0006" to •0192" in length, v.v.
/3. V. oval or broadly elliptical, v.v.
y. Frustule deeply geniculate, v.s.
Ag. Syst. p. 1. Ag. Consp. p. 58. Eng. Bot. 2nd ed. pi. 2560. Grev. B .F.
p. 404. Harv. Man. p. 200. Ehr. Inf. xx. 1. Ralfs, Ann. vol. 13.
pi. xiv. 7. ad specim. authen. in Herb. Jenn. Kiitz. Bacill. xx. 1.
Quek. H. C. xii. 8. Conferva stipitata, Eng. Bot. 1st ed. pi. 2488.
Achnanthes Carmichaelii, Grev. B. F. p. 404. Harv. Man. p. 200. Kiitz.
Bacill. XX. 2. Achnanthes hrevipes, Kiitz. Bacill. xx. 9.
Marine. Parasitic on other Algæ. Poole Bay, June, and in conjugation,
July 1849. Jersey, July 1852. Larne, Ireland, July 1853. Newhaven,
Sussex, Oct. 1853, W.Sm. Southampton, April 1844, Mr. Jenner. Southampton,
July 1812, Miss Hill Miss Biddulph-, communicated by Mrs.
Griffiths. Var. ft. Seaford, Sussex, April and Sept. 1851, and in conjugation,
March 1852, W. Sm. Var. y. Newhaven, Sussex, July 1843, Mr. Jenner.
(Mediterranean near Marseilles. Salt Pits near Agde, May 1854, W. Sm.)
I have included under the above both A. Carmichaelii, Grev., and A. hrevipes,
Kiitz., neither species, judging from the figures in the ‘ Bacillarien,’ having
any distinctive characters save the greater or less length of the stipes ; while
in all my various specimens this feature is of the most variable kind, frustules
nearly sessile, and others with a stipes ten times their own length occurring in
the same cluster. With regard to Var. ft. I am in doubt whether it should
not be ranked as distinct, the broadly elliptical form of the frustule being
tolerably constant, and the habit of growth more diffuse and scattered than in
my other specimens. It has also maintained its characters unchanged for
several successive seasons, disappearing and being again produced in the same
pool without any apparent change in its form or habit ; at the same time,
forms intermediate between it and the most linear and clustered specimens of
A. longipes have so often presented themselves in the same and other localities,
that I do not feel myself warranted in elevating the Seaford plant into a
distinct species.
The presence of costæ is alone sufficient to distinguish A. longipes and its
varieties from the other species of this genus. These costæ appear to be
thickenings on the under surface of the siliceous valve, and not, as in others
of the Diatomaceæ, confluent beaded striæ ; the latter extending, as is ordinarily
the case, over the entire surface of the valve, though the regularity of
their arrangement is much obscured in the present species by the interference
produced by the underlying costæ.
Plate XXXV. 300. Plate XXXVI. 300. Var. ft. Plate XXXVI. 300 0
Var. y. Plate XXXVI. 300 y.
2. Achnanthes hrevipes, Ag. Filaments containing from 2 to
several hundred frustules ; V. linear or oval, attenuated or acute.