PROCEEDINGS OF SOCIETIES. 127 
be assumed to be the source of the Arachnoidiscus Ehrenberghu 
noticed by him as occurring in a gathering made at Malahide, 
Co. Dublin, in December, 1864, and recorded in the Minutes of 
the Club for that month, and this for the conclusive reasons then 
advanced. (See ‘Quart. Journ. Mic. Science,’ vol. xii, p. 1382 
and p. 167.) 
Mr. Archer showed a minute alga which formed a new species 
of the genus Dictyospherium, Nig.; and, for the sake of com- 
parison, he showed along with it the tolerably common but 
remarkable little plant, Dictyospherium Ehrenbergianum, Nag. 
Before, however, describing the present form, inasmuch as the 
genus Dictyospherium, as well as many others of the “ unicel- 
lular” alge (quantum valeant) have been ignored by some—for 
instance, the authors of the ‘ Micrographic Dictionary’—it might 
be well here as briefly as possible to give the generic characters, 
following as closely as possible Nageli’s own words (‘ Gattungen 
einzelliger Algen,’ p. 72) :—* Cells elliptic, with thick confluent 
mucous investment, combined in numbers into free-swimming, 
one-layered, hollow-globular (microscopic) families, one always at 
the ends of delicate threads which proceed from the central point 
of the family and which become repeatedly branched towards the 
periphery ; division at the commencement of a series of generations 
in all directions of space; afterwards, as regards the middle point 
of the aggregate family, as arule, alternating only in the two 
tangental directions.” As will be presently seen, the three forms 
otherwise referable to this genus, possessing cells which in each 
are respectively elliptic, reniform, and constricted, renders it 
necessary that the foregoing characters be modified so far as 
relates to this particular. In this palmellacean genus the cells 
form little definite families or colonies primarily originating from 
a single cell by constant division, each new cell being supported 
on the summit of a slender thread or fibre-like stipes of extreme 
delicacy, which thus during the increase in number of the consti- 
tuent cells of the family becomes usually dichotomously divided, 
the whole family being imbedded in a definitely bounded mucous 
or gelatinous investment. The form on which the genus was 
founded by Nageli (Dictyospherium Ehrenbergianum, Nag.) is 
very minute, and, in suitable places, common, the families in the 
ageregate forming a globular or broadly elliptic, or sometimes 
subcubical figure; the rate of growth of the delicate thread being 
equal all round, the cells at the ends of each of its dichotomous 
ramifications stand at nearly equal distance from the original 
centre—hence the regular figure of the aggregate family. In this 
species the individual cells are elliptic, as in Nageli’s original 
diagnosis of the genus founded on this as the then only known 
type, and are very minute. Another species in this genus has 
been described by Rabenhorst (in ‘ Kryptogamen-Flora von 
Sachsen,’ &c., p. 132) under the name of Dictyospherium rent- 
‘orme (Bulnheim in Hedwigia, 1859). This plant possesses larger 
VOL. VII.—NEW SER. K 
