reduce it to some rule by assigning to I. nervosa all species which
liave been described as “ striated though still in doubt whether the
arrangement of the cellules may not in some cases, under imperfect
instruments, have been regarded as “ striation.”
1. Isthmia nervosa, Küts. Valves inflected towards the suture,
transversely lined by costæ which anastomose and disappear towards
the centre of valve ; cellules of connecting-membrane much
enlarged near the suture. Length of valve -0024" to -0082". v.s.
Kiitz. Bacill. xix. 5. Isthmia ohliqnata, Ag. Consp. p. 55. Ehr. Inf. xvi. 5.
Ralfs, Ann. vol. 12. viii. 2. Harv. Man. p. 200. Biatoma obliquatum,
Lyng. Tent. ixii. c. 2.
Marine. Carnlough Bay, North of Ireland, Mr. D. Moore ; communicated
by Mr. Jenner. “ Near Gleuarne,” 1847, Mrs. Lyon-, communicated by
Dr. Landsborough. Balta Sound, Mr. Edmonstone, communicated by Mrs.
Griffiths.
Plate XLVII.
2. Isthmia eneirrìs, Ehr. Valves direct, not inflected; cellules of
connecting-membrane slightly enlarged at the suture. Length of
valve '0024" to ’0082". v.v.
Ehr. Inf. xvi. 6. Ralfs, Ann. vol. 12. viii. 1. Kiitz. Bacill. xix. 4. Prit.
Anim. iv. 183. Isthmia obliquata, ft. tenuior, Ag. Consp. p. 55. Conferva
obliquata, Eng. Bot. tab. 1869.
Marine. Torbay, July 1846; Cork Harbour, Aug. 1855, W.Sm. Torbay,
Mrs. Gi-iffiths. Ilfracombe, Mr. Ralfs. Maiden Rocks, near Larne,
Dr. Dickie. Haverfordwest, July 1854, Mr.
Plate XLVIII.
G e n u s 48. PODOSIRA, Ehr.
Filament cylindrical, of few frustules, attached ; frustules spherical or
cylindrical, geminate by the persistence of the connecting-membrane
as a siliceous cingulum ; valves hemispherical, minutely
punctate or cellulate ; silex absent from the apex of the valve.
Having employed the word “ annulus” in another connexion, I
adopt the term “ cingulum” in the present and following genus to
denote the subpersistent condition of the connecting-membrane,
present in all the Melosirees, but eminently conspicuous in Podosira
and Melosira, retaining the frustules in these genera, after self-division,
in a geminate union until the self-dividing process is renewed.
The brevity of the filaments, which rarely include more than from
two to six frustules, the absence of silex at the apex of the valves,
and its parasitic growdh, are characteristics which place the genus
Podosira apart from its allies. This deficiency of silex is probably to
allow a free secretion of the mucus which unites the frustules, and
provides a pedicel for their attachment to the plant on which they
grow, as it does not occur in the non-attached valve of the first-
formed frustule. In the living state, the absence of the silex is not
perceived ; but when the frustules have been macerated in acid, these
portions of the valves appear as perforations, owing to the disappearance
of the cell-membrane. A similar secretion of mucus takes place
in Melosira nummuloides ¡3. (Plate XL IX . 329 ft.) ; but in this case no
deficiency of silex is apparent, and no perforations are seen in any
condition of the valves : tiie cause of this diversity of structure,
under circumstances so similar, I cannot explain.
1. Podosira Montagnei, ATiite. Filaments usually of two frustules,
which are at first compressed at the equator; cingulum firmly
siliceous, with annulate striæ. Breadth of filament -0018" to
•0028". v.v.
Kiitz. Bacill. xxLx. 85. Melosira gloUfera, Harv. Man. p. 196? Ralfs,
Ann. vol. 12. ix, 3. ad specim. authen. in herb. Jenn. Rosaría qlobi-
fera, Carm. apud Grev. B. F. p. 372 ?
Marine. Hastings, Mr. Jenner. Torbay, July 1846 ; Poole Bay, Nov.
1849 and Sept. 1851, W. Sm. (Near Jaffa, Levant; communicated by Mr.
Johnson.)
Plate XLIX. 326.
2. Podosira hormoides, Kiitz. Filaments of from two to six
frustules, which are at first compressed at the poles; cingulum
obscurely punctate. Breadth of filament -0007" to -0009". v.v.
Kiitz. Bacill. xxviii. 5. & xxix. 84. Prit. Anim. xiii. 45.
Marine. Pevensey Bay, Sussex, Aug. 1850; Poole Bay, Sept. 1852;
Stomach of Pecten, Coast of Sussex, Mar. 1853, &c., W. Sm. Frith of Clyde,
Feb. 1854, Mr. Ilennedy.
Plate XLIX. 327.