G e n u s 27. PODOSPHENIA, Ehr.
Frustules attached, sessile, cuneate ; valves convex, obovate, inflected
at the larger extremity, striated, traversed by a longitudinal median
line.
This genns is placed by Professor Kützing at the head of his second
tribe, viz. Diatomeoe vittatæ. The Vitta, upon the presence of which
he has ' constituted this division, do not seem to me to be special
organs, but modifications in the outline of the valve, which in the
present genus is slightly inflected at its larger extremity, causing on
a front view the appearance of notches at the spot where the valves
unite with the connecting membrane, and the foramina exist. The
apparent prolongation of this notch to the lower extremity of the frustule,
is nothing more than the valvular suture which is seen in all the
Diatomaceæ. The appearance of two notches at one extremity in the
same frustule arises from the progress of self-division ; for on the
first formation of the new half frustules, the inflected portions of each
valve are in close proximity, and but one notch and one “ vitta” can
be detected. An examination of the frustules, given in Plate XXIV.
figs. 225 and 229, will show the gradual progress of self-division,
and the various appearances consequent thereupon. A side view
of a single valve after maceration in acid, as in PL XXIV. 225 a,
shows the true form of the valve, and the outline of the inflected portion.
We have here the first stage in a modification of the form of
the valve, which, in its more complete development in TabeUaria and
Grammatophora, will remind us of the binate form of many of the
Desmidieæ, in which the loculi of the cell are united by a narrow
connecting isthmus.
The only very evident distinction between the present and succeed-
genus, is the presence in Rhipidophora of an elongated and frequently
dichotomous stipes ; but a close examination of the frustules
shows us that the distinct and even moniliform striæ, so conspicuous
in Podosphenia, are almost wholly wanting in our native species of
Rhipidophora.
1. Podosphenia Ehrenbergii, Kiitz. P. V. truncate at the upper
extremity; V. acute at both extremities; striæ moniliform, 27 in
•001". Length -0025" to -0055". v.v.
Iti; I.
Kütz. Bacill. ix. 13. Prit. Anim. xvi. 14. Podosphenia mneata, Ehr. Inf.
xvii. 8.
Marine. S h o r e h a m H a r b o u r , Aug. 1850, IF. Sm. Torhay, Mrs. Griffiths.
Plate XXIV. 225.
2. Podosphenia ovata, n. sp. F. V. rounded at the upper extremity
; V. rounded at upper, and acute at lower extremity ; striæ
moniliform, 24 in -OOl". Length -0033" to -0042". v.v.
Marine. Shoreham Harbour, Aug. 1850, W. Sm.
Plate XXIV. 226.
3. Podosphenia Lyngbyei, Kütz. F. V. truncate at upper extremity
; V. rounded at upper, and acute at lower extremity ; striæ
delicate, 46 i n -001". Length-OOll" t o -0033". v.v.
Kütz. Bacill. X. 1, 2. Prit. Anim. x v i . 13. Podosphenia abbreviata, Ehr.
Inf. xvii. 7.
Marine. Coast of Sussex, frequent ; June and Aug. 1850, W. Sm.
Plate XXIV. 227.
4. Podosphenia Jurgensii, Kütz. F. V. truncate at upper extremity;
V. rounded at upper, and acute at lower extremity ; striæ
very faint, 48 in -OOl". Length -0025" to •0031". v.s.
Kütz. Bacill. ix. 12. Styllaria cuneata, Ag. in Grev. B. F. p. 408. ad specim.
authen. quæ communicavit cl. Dr. Greville.
Marine. Torbay, Mr. Balfs.
Plate XXV. 228.
5. Podosphenia gracilis, Ehr. F .V . almost linear, truncate; V.
scarcely siliceous ; striæ obscure. Length •OOll" to •0021". v.v.
Ehr. Inf. xvii. 6. Kütz. Bacill. ix. 10.
Marine. Jersey, Aug. 1852, IV. Sm.
Plate XXIV. 229.
G 2