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I
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This species differs so considerably in the form of the
marginal cells, that it is difficult to conceive it to be merely
a variety of that species. None of Ehrenberg’s figures
come near to it. PI. xi. fig. 5. d. E . represent distinct species
of Pediastrum, which have not been described as such, and
the first of which might be named Pediastrum crihriforme,
and the second P. lunare. See Plate x c il. figs. 3, 4. Several
other species of Pediastra are represented in Ehrenberg’s
figures, and confounded with other species. These I
propose to figure in an appendix to this genus, in the hope
tha t they may attract the attention of those who study the
beautiful tribe of Desmideæ,
65. SCENEDESMUS Meyen.
Char. Frond composed o f from two to ten cylindrical, fu s iform
or ohlong cells disposed in one or tioo series, outer
ones often lunate.
Derivation. From aKyvy, a tent, and Ssagos, a bond.
Arthrodesmus Ehrenherg, in part.
1. SCENEDESMUS QUADEICAUDATUS Breb.
Plate X C II. Fig. 12.
Char. Cells usually four, but sometimes there are eight in
each frond, rounded at the ends, disposed in a single
series, each extremity o f the two external cells prolonged
into a bristle.
fl Cells rather small, external ones with a bristle at each
extremity, and one at the outer margin.
7 ecornis Ehr. — E xternal cells without bristles.
Breb. Alg. Falaise, p. 66. Menegh. Syn. Desmid. in Lin-
naaa, 1840, p. 206. Arthrodesmus quadricaudatus Ehr.,
Infus. p. 150. t. 10. fig. 16.; Pritch. Infus. p. 189.;
Ilalfs, in Annals, vol. xv. pi. xii. fig. 4 .; Jenner, in FI.
of Tunbridge Wells, p. 200. — g ecornis. Scenedesmus
Leibleini Kiitz., Synops. Desmid. in Linnsea, 1833,
p. 607. fig. 98. ; Menegh., 1. c. p. 207. Scenedesmus
bijugatus, trijugatus, and minor Kiitz., 1. c. p. 607. figs.
97. 99.; Jenner, 1. c. p. 200.
Plab. Storrington Common and Eastbourn, S u ssex ;
Shoreham, K e n t: Mr. Jenner. Cheshunt: A. PL. H.
Dolgelly : Mr. Ralfs. — fl. Dolgelly : Mr. Ralfs. —
7. Weston Bogs, near Southampton: Mr. Jenner.
B ris to l: Mr. Thwaites. Dolgelly, and Penzance : Mr.
Ralfs.
The cells in this species are quadrangular, about three
times as long as broad, and rounded slightly at their extremities
; the external ones are usually more turgid, and the
bristles at their extremities are directed outwards. The
variety fl and 7 are figured by Ehrenberg.
2. Scenedesmus dimorphus Kütz.
Plate X C I I. Fig. 13.
Char. Cells from fo u r to eight in each frond, arranged in a
single series. Inner cells fusiform, attenuated at each
end; outer externally lunate.
Achmanthis dimorpha and bilunulata Turpin, Mem. du
Mus. xvi. p. 309. pi. xiii. f. 11, 12, 1828. Scenedesmus
pectinatus Meyen, Nov. Act. Nat. Cur. xiv. p. 375.
tab. 43. f. 34. 1829. Scenedesmus hilunidatus, dimorphus,
pectinatus Kütz., Synops. Diat. LInnasa, 1833. 608.
t. xix. fig. 93.; Ehr. Die Infus. p. 150. tab. x. fig. 17. ;
Kalfs, in Annals, p. 403. pi. xii. fig. 5.
ELah. Dolgelly ; Mr. Ralfs. Near B ris to l: Mr. Thwaites.
This is a very distinct and pretty species.
3. Scenedesmus acutus Meyen.
Plate X C I I. Fig. 14.
Char. “ Cells two to six, fusiform, acute at both ends, un