qitaih'angular, each angle produced into a spine. End
view subelliptic, zoith a spine at each extremitg.
Artiirodesmus octocornis Ehr., Infus. p. 152. S.? octocorne
Kalfs, in Annals, vol. xv. p. 159. pi. xii. fig. 3.
Ilab. Boggy ponds near Dolgelly, rare ; 3Ir. Ralfs.
“ The newly formed segments at first have only two
spines, and in this state somewhat resemble those of Staiir-
astrum Incus, of which indeed this plant may eventually
prove a variety. B u t S. Incus has only two spines on each
segment, and its end is not concave hut truncate.” — Ralfs.
The AHcrasterias octocornis of Meneghini would appear to
he a different plant.
56. X A N T H ID IU M Ehr.
Char. Frond comjwsed o f two slightlg compressed segments
constricted in the centre, neither lobed, sinuated, nor emarginate.
Surface clothed loitli simple or branched elongated
spines, either scattered or confined to the margin,
where theg are apparently placed in two rotes, one on each
side the marginal lines. Segments anteriorly perforated
in the centre (?).
Derivation. From ^avdos, yellow ?
This genus bears a close resemblance to Cosmarium, from
which there is no satisfactory character to distinguish it,
unless indeed the projection or aperture noticed to occur by
Mr. Dalrymple in Xanthidium furcatum, and by Mr. Ralfs
in X . aculeatum be constant in all the true species of the
genus Xanthidium. Meneghini even goes so far in the union
of the genera Xanthidium and Cosmarium, as to retain in the
former genus only those species whose spines are scattered
over the surface, while he refers to the latter genus those in
which the spines are confined to the margin. This arrangement
seems arbitrary ; and that it is so, is proved by the fact
that in a species of Xa.nthidium figured hy Dr. Bailey in the
“ American Bacillaria,” and which has six pairs of marginal
spines, the central apertures exist ; that is, in the species referred
by Meneghini to Cosmarium these openings are met
with — the only character which is even supposed to belong
exclusively to Xanthidium.
“ I have little doubt that there are more British species of
Xanthidium than I shall here describe, and indeed believe
that I have more than once gathered X . hirsutum ; but as
my attention was not directed to the Desmideæ at the time,
I neglected to examine it with sufiicient care to be able to
state my observations with any confidence in this paper.”
Ralfs.
1. X a n t h i d i u m f u r c a t u m Ehr.
Plate L X X X IX . Fig. 1.
Char. Segments reniform. Spines at fir s t marginal, subsequently
scattered, dividing at their terminations into
three or fo u r acute divergent points.
Xanthidium furcatum Ehr., Infus. p. 148. tab. x. f. xxv. ;
Menegh. Synops. Desmid. p. 224. Xanthidium No. 2.,
Bailey’s American Bacillaria, p. 291. pi. i. fig. 15. ;
Ralfs, iuAunals, vol. xiv. p. 466. pi. xii. fig. 1.
Hab. Several places in Sussex : Mr. Jenner. Dolgelly,
and near Carmarthen: ilfr. Ralfs. High Beech and
Cheshunt Common : A. II. H. Aberdeen : Dr. Dichie.
Ireland : Mr. Moore.
This is the finest as well as commonest species of the genus.
There is no other with which it can be confounded.
In the figure of this species, given by Ehrenherg, the «entrai
aperture seems to he indicated.
2 . X a n t h i d i u m f a s c i c u l a t u m Ehr.
Plate L X X X IX . Pig. 2.
Char. Segments small, reniform. Spines in pairs, and
usually six pairs to each segment.
Xanthidium aculeatum, Abhandl. d. Akademie d. IVissensch.
A A 4