272 Proceedings of the Royal Irish Academy. 
being only a mucous cushion or stipes, and on these grounds the genus 
Isthmia ought to be excluded. In the case of Hydrosera ( Wallich), 
the frustules are not symmetrical, processes occurring on the one valve, 
and not on the opposite one. Wallich’s description is ‘‘on one side 
only, with a remarkable series of aperture-like appendages.’”’? Wallich 
on Triceratium, Q.J.M.8., July, 1858, p. 251. For which reason I 
consider the genus Hydrosera is not properly comprehended in the 
group. ‘The species marked by an angular outline of the valves as 
Triceratium, Amphitetras, &c., however closely related to the Biddul- 
phie, seem however to possess such distinctive peculiarities of struc- 
ture as to justify their being placed in a separate group; and if any 
forms of the genus Hemiaulus had occurred in Irish localities, I would 
have been disposed to include them with the Biddulphiez as Raben- 
horst has done in his Flora Europea Algarum. 
Various generic names have from time to time been introduced by 
different writers to designate the forms of this group, in consequence 
of which much confusion has arisen, to obviate which a few remarks 
are here necessary. 
The generic name Biddulphia was first adopted by Gray, and 
along with Biddulphia pulchella embraced some heterogeneous forms, 
which latter were afterwards removed to their proper places. Agardh 
then established the genus Odontella to receive the single species now 
known as Biddulphia aurita; Ehrenberg having appled the name 
Odontella to a species of Desmid, as Roper informs us, Q.J.M.S., 
Oct., 1858, p. 8, substituted for it the designation Denticella, which 
was thus equivalent to Agardh’s Odontella. The forms included in 
these genera, Biddulphia and Denticella, were filamentous; and 
Ehrenberg having found kindred forms which, without sufficient 
examination, he considered to be simple, adopted the genera Zygoceros 
and Cerataulus, the former for those free forms, as he thought them 
allied to Biddulphia, the latter to Denticella. Some of these genera 
have been retained by succeeding writers, but Smith in his Synopsis 
has, as I think, wisely dispensed with these superfluous subdivisions, 
and included the forms contained in them under the one generic 
name. 
Rabenhorst, in his Flora Europea Algarum, places the Biddulphiese 
in close connexion with the septate forms, supposing, as I imagine, that 
the coste on the valves of Bid. pulchella and other species with un- 
dulate surfaces are septa. On this subject the observations of Smith 
are worthy of notice: ‘‘The existence of septa in B. pulchella is by 
no means to be admitted, though the costs may occasionally project 
into the interior of the cells.” B.D., Vol. 11., p. 49. 
Genus I. Brppvrpnta, Gray. 
Processes projecting outwards at a more or less acute angle from 
the plane of the base. 
