174, 
Description of Two NEW sPEcrIES of Cosmarium (Corda) of 
Penium (Bréb.) and of Arturopesmus (Ehr.). By 
Wiuiam ARCHER. 
(Read before the Natural History Society of Dublin.) 
Family DESMIDIACE. 
Genus CosmMaritum, Corda. 
CosMARIUM PYGMa@UM, mihi, sp. noy. 
Specific characters—Frond very minute, smooth, seg- 
ments sub-quadrilateral; end view sub-elliptic, somewhat 
inflated at each side at the centre. 
Locality.—Featherbed Bog and elsewhere in pools in Dublin 
mountains; not very rare; on submerged plants, and in 
Sphagnum pools, coating the moss. / 
General description —Frond very minute, smooth, rather 
broader than long; constriction a minute linear acute notch ; 
segments in front view rather more than twice as broad as 
long, subquadrilateral, outline sometimes slightly irregular, 
ends straight or slightly curved; side view scarcely twice as 
long as broad, constriction a triangular emargination on each 
side, segments orbicular ; end view sub-elliptic, with a gentle 
gradual central protuberance at each side, extremities sub- 
conical, rather abruptly rounded. Zygospore orbicular, 
smooth (without spines), placed between the shortly decidu- 
ous empty parent fronds. 
Measurements.—Length of frond, ~,; to 334; breadth, 
sone to +155; depth (thickness), ~4,,5 to =355 of amanel 
Pl. VI, Figs. 45, 46, front view; 4:7, side view; 48, end 
view of frond; 49, zygospore. 
Affinities and differences.—There is no danger of mistaking 
this for any other species, except, perhaps, C. tinctum; but 
from it, this form is at once distinguished by its subquadri- 
lateral, not broadly and regularly elliptic, segments, and by 
its colourless, not faintly reddish, cell-wall. I have met this 
form for two or three years, and I consider it is perfectly 
distinct, nor is there any other species with which it need be 
contrasted. There is a slight variation as to size within the 
limits above mentioned. I might remark that the general 
contour of the segments seems to me to resemble in some 
degree those of Cosmarium biretum (Bréb.), and which to 
those acquainted with that species may serve to help out my 
