6. Himantidium gracile, Ehr. Valve arcuate or direct, linear; extremities
rounded, somewhat inflated or slightly recurved ; striæ
27 in -001". Length of frustule -0035" to -0065". Breadth of
valve -0003". v.v.
Kûtz. Bacill. xxix. 40. ad specim. quæ dedit auc. De Brébisson.
Fresh water. Killicraukie, Mr. P. Grant. Dolgelly Earth. Lough Mourne,
Lough Island Reavey, and Mull Deposits. (Fountain at Nismes ; and Pic
du Capucin, Auvergne, elev. 4567 feet, June 1854, W. Sm.)
Plate XXXIII. 285.
7 . Himantidium majus, W. Sm. V. arcuate, dorsum elevated,
extremities inflated and rounded ; striæ 27 in '001". Length of
F. ’0055" to '0075". Breadth of valve ‘0006". v.s.
|S. with two dorsal ridges.
fi. Himantidium hidens, Greg. M. J. vol. 2. iv. 21.
Fresh water. Gap of Dunloe, Killarney, July 1855, W. Sm. Forfarshire,
Dr. Dickie. Braemar, Dr. Balfour, Aug. 1854. Var. fi. Mull Deposit,
Dolgelly Earth, &c.
Differs little from H. gracile, save in its greater size and elevated dorsum,
and is probably a sporangial form of it, or some other species.
Plate XXXIII. 286. Var. fi. Supp. Plate LX. 286 fi.
S e c t . I I I . Filaments tenacious : marine.
8 . Himantidium ? Williamsonii, n. sp. Frustules direct ; F. V.
■with central, terminal, and two intermediate inflations ; central
inflation with an acute angle, the others rounded ; valves striated ;
striæ 20 in 'OOl".
Marine. Dredged off the Island of Skye by Mr. G. Barlee ; communicated
by Prof. Williamson.
I have not been able, in the few specimens I possess of this interesting
form, to obtain a S. V. of the frustules, and am in consequence in some doubt
whether it might not be referred to another genus, probably to Biddnlphia,
to which the inflations on the surface of the valve and its marine habitat
seem to ally it. The general appearance and tenacity of the filament are,
however, those of the present genus, with which it must remain associated
until a further examination confirms its present position or necessitates its
removal.
Plate XXXIII. 287.
G e n u s 3 3 . ODO NTID IUM , Kütz.
Frusinles quadrangular, direct, united into a filament, at first attached,
at length free ; valves elliptical or cruciform, costate, costæ pervious
or interrupted.
The frustules of Odoniidium are distinguished from those of the last
genus by their straight outline and conspicuous costæ, from those of
Fragilaria by the latter character, and from those of Henticula by
their more tenacious adherence and striated valves. I t must however
be acknowledged that there is little to separate the last-named genus
from our present one, and I should be disposed to unite the two,
were there not in the general habit of the living frustule characteristics
which enable the observer to assign them to their respective
genera; Odontidium often occurring in filaments of considerable
length, and in some cases of no little tenacity ; Denticula invariably
in fragmentary portions rarely presenting more than two or four in
union, and that union apparently of the feeblest kind. The surface
of the valve in Denticula is also very distinctly striated, while in
Odontidium the striæ, if present, are obscure, and elude detection by
all but the most powerful and carefully-adjusted glasses. The
appearance of minute teeth on the F. V. of the frustule, which arises
from the terminations of the costæ, and from which Professor Kützing
has derived the generic appellation of Denticula, is however far more
conspicuous in some of the species belonging to Odontidium ; and
the two genera are so alike in their leading features, and their
appellations so similar in meaning, that it may be left to future
observers to consider whether they may not without much inconvenience
be permanently united.
1. Odontidium h y em a le ,/Gite. V. elliptical, frequently attenuated
towards the obtuse extremities; costæ pervious, from 5 to 12.
Length of frustule -0008" to •0014".
Kütz. Bacill. xvii. 4. Rahen. Süssw. Diat. ii. 4. Fragilaria hyemalis, Lyng.
Tent, ixiii. E? Ralfs, Ann. vol. 12. pi. Ü. 5. ex parte, et Hass. Alg.
xcv. 5. ex parte, ad specim. authen. in herb. Jenn. Fragilaria Con-
fervoides, Grev. B. P. p. 403. ad specim. authen. in herb. Grev. Harv.
Man. p. 197. Odontidium turgidulum, Kütz. Bacill. xvü. 2. Raben.
Süssw. Diat. ii. 6. Fragilaria twgidula, Ehr. Inf. xv. 13.