Roper, on Biddulphia. 
3 
species of tlie genus, whicli is so exceedingly variable in 
form, that it has led to many of the errors which have been 
propounded by subsequent observers. Agardh, in his 
' Systema Algarum/ in 1824, and afterwards in his ' Con- 
spectus Criticus Diatomaceorum,^ published in 1830, was 
the first who brought forward anything approaching a full 
classification of this tribe of plants, with an extended list of 
species ; but he appears to have been ignorant of the classi- 
fication of Gray, in his earlier work ; and though he alludes to 
Biddulphia in the second, the species pulchella was unknown 
to him except by description, and he continues to retain it 
under the title of Diatoma BiddulpManum, and states that it 
probably may prove one of the Desmidiese. In the same work, 
however, probably from having seen specimens, he appreciates 
the characteristic distinction between the D. auritum of 
Lyngbye and the true Diatomas of De CandoUe, and founded 
the genus Odontella for its reception. M. De Brebisson, in 
his ' Considerations sur les Diatomees,^ in 1838, having had 
his attention directed for some time to this class of Algae, 
and aided by improved instruments, was able to make a con- 
siderable advance on the classification of Agardh, and adopt- 
ing Gray^s genus of Biddulphia, with a true appreciation of 
specific characters, associated with it the Odontella auritum 
of the former writer; and the same arrangement was 
adopted by Mr. Balfs, apparently from independent observa- 
tion, in 1843."^ In this arrangement, unfortunately, these 
writers were not followed by other Continental observers ; 
Kiitzing retaining the genus Odontella in his latest works ; 
whilst Ehrenberg, having in his ' Infasionsthierchen^ applied 
that name to a species of the Desmidiese, proposed that of 
Denticella for the same forms. In 1839 the latter writer 
communicated to the Berlin Academy a paper on the marine 
species of Diatomacese found at Cuxhaven, in which, among 
other new genera, he described one, under the name of 
Zygoceros, to contain those species of the Biddulphian type 
which he considered to be free forms, and not concatenate ; 
but at present we have only negative evidence in support of 
this being a permanent distinctive character, and, as Professor 
Smith has observed in describing his Biddulphia rhombus, the 
typical species of the new genus, " the form and structure are 
too near those of Biddulphia to permit its separation, and the 
filamentous condition of the species will no doubt reward the 
future explorers of the tidal harbours and estuaries of Britain/^f 
In 1843, Ehrenberg added another genus under the name 
of Cerataulus to include those species allied in form to his 
* ' An. Nat. Hist.,' vol. xii, p. 273. 
f * Synopsis/ vol. ii, p. 50. 
