3. Podosira? maculata, TF. Sm. Dry valve purplish, with alternate
bands of shading, distinctly punctate.
Marine : not unfrequent in deep water. Stomach of Pecten, very abundant,
Coast of Sussex, March 1853 ; Stomacii of Pecten, Kiusale Bay, April
18.53, &c., jr. Sm. Wisbeacli, Dec. 1853, Mr. S. Smith. Frith of Clyde,
Feb. 1854, Mr. Hennedy. Pembroke Harbour and Thames, Mr. Itoper.
Newbie Deposit, Prof. Harkness. (Cherbourg, M. Be Brébisson.)
Having never been able to detect tliis species in a fresh state, I cannot refer
it with confidence to the present genus ; its loose valves are, however, so
similar to those of P. Montagnei, that I have felt constrained, in the absence
of fuller evidence, to place it near that species. Cyclotella Scotica, Kiitz.
Bacill. plate i. fig. 2 & 3, is also exceedingly similar, and may prove to be the
present species.
Plate XLIX. 328.
G e n u s 49. MELOSIRA, Ag.
Filament cylindrical, of numerous frustules, attached or free ; frustules
spherical or cylindrical, often geminate ; valves hemispherical
or subcylindrical, more or less convex at the junction-surfaces.
In an admirable paper on the conjugation of the Diatomaceæ,
inserted in the ‘Annals of Natural H isto ry ’ for March 1848, Mr.
Thwaites suggests the propriety of separating from the genus Melosira
all those species “ whose frustules are not convex at their extremities,
and which form by their close contact an uninterrupted cylindrical
filament.” To the genus thus formed he gave the name of Orthosira :
this suggestion I have adopted in the present work.
Mr. Thwaites also proposes to construct a genus, “Aulacosira,”
for the reception of such species “ as are characterized by the absence
in the frustule of an evident central line (suture) indicating the place
of future fissiparous division, but each frustule having two somewhat
distinct sulci or fossulæ passing round it.” This genus I cannot
adopt, as I believe the characters have no real existence, and owe
their apparent presence in the species Mr. Thwaites adopted as his
type, viz. Melosira crenulata, Kiitz., only to accident, or observation
under imperfect illumination. A careful study of the specimens from
Aberdeen, upon which Mr. Thwaites himself founded ids remarks,
and gatherings from various other localities, has failed to satisfy me
that any essential differences exist between this species and other
Orthosira;. The “ crenulations,” noticed by Kützing and Thwaites,
are probably owing to a slight inflection at the suture (in younger
specimens) of the margins of the valves : even this inflection is, I fear,
only apparent, and dependent mainly upon the adjustment of the
light, as Mr. West gives little indications of it in his drawings, which
are executed with scrupulous fidelity.
Adopting the genus Orthosira, I am therefore constrained to
reject that of Aulacosira, and divide the species formerly included
under the common name of Melosira into two genera; the first, Melosira,
known hy the more or less rounded outline of the valves, and
the geminate union in its frustules ; and the other, Orthosira, hy
having the junction-surfaces of adjacent valves plane, or truncated,
the line of junction usually marked hy circlets of denticulations or
spines, and the frustules continuously united.
I t is probable that differences exist in the formation of the sporangia
of the tw'O genera ; as these bodies, very frequent in Melosira, are of
rare occurrence in the other genus, and present peculiarities in the
single native species in which they occur, which would seem to indicate
specialties in the function concerned in their production. These
peculiarities will be noticed elsewhere, and are not, I fear, in the present
state of our knowledge, available as the groundwork of a correct
classification.
1. Melosira nummuloides, Kütz. Ordinary frustules globular;
sporangial frustules spherical, w'ith the polar diameter shorter than
the equatorial, all geminate ; valves hemispherical, furnished with
a subcentral keel or flange of silex. Breadth of ordinary filament
-0005" to ’OOOS". Breadth of sporangial filament '0011" to
•0013". v.v.
Var. ft. Mucus-cushion much developed.
Var. y. Keel often obscure, or absent.
Kiitz. Bacill. iii. 3. ad specim. quæ dedit cl. De Brébisson. Grev. B. F. p. 401.
Ralfs, Ami. vol. 12. pl. ix. 1. GalUonellanmnmxdoides, Ehr. Iiif. x. 3.
&xi. 1. Prit. Anim. xiv. 14. Fragilaria nummuloides, Ljw^.'Fcwt.
ixiii. c. Conferva nummuloides, Dillw. Coiif. p. 43. tab. B. Melosira
moniliformis, Ag. Syst. p. 8 . ad spec, authen. in herb. Grev. Melosira
satina, Kütz. Bacill. iii. 4. sic cl. Auctor in lit. Jul. 1851. A’ar. ft. Melosira
salina', ft. concatenata, Kütz. Bacill. iii. 5. & xxi. 5 b.