ARCHER, ON TWO NEW SPECIES OF COSMARIUM. 175 
description ; but it would be simply absurd to institute any 
further comparison between them. 
The orbicular smooth zygospore without spines seems fur- 
ther to indicate an affinity with C. tinctum. This latter I 
have moreover found to possess orbicular, not quadrate zygo- 
spores—thus, perhaps, pointing to an affinity between these 
species more strongly. But the parent forms are readily dis- 
tinguished by the characteristics alluded to above; and I 
cannot fancy the possibility of their being confounded. Thus 
this distinct little species forms an additional exception to 
the generally pervading rule, that in this family the deeply 
constricted short forms possess spinous zygospores. But I 
shall take a future opportunity to draw attention to a few 
cases of exceptional form as regards the presence or absence 
of spines in the zygospores of the Desmidiacez, 
Genus ARTHRODESMUS, Ehr. 
ARTHRODESMUS TENUISSIMUS, mihi, sp. nov. 
Specific characters.—Frond extremely minute; segments 
subhexagonal, opposite lateral extremities acutely cuspidate, 
each upper angle furnished on each front with a minute 
acute mucro, which four in the fusiform end view stand out 
divergently. 
Locality—Featherbed Bog, in Sphagnum pools, coating 
the moss. 
General description.—Frond very minute, smooth, some- 
what broader than long; constriction, a rounded or bluntly 
triangular sinus; segments in front view about twice as broad 
as long, sub-hexagonal, the sides somewhat concave, the oppo- 
site lateral extremities projecting horizontally, acutely cuspi- 
date, ends truncate or slightly concave, the two upper angles 
at each front view furnished with a minute spine or mucro, 
but which, in this view, being turned rather towards the 
observer than divergent, each appears as a minute somewhat 
opaque thickening at the angles; frond in side view about 
twice as long as broad, oblong; constriction, a shallow emar- 
gination, ends broadly rounded, each furnished at each 
opposite side with a single short acute divergent mucro; end 
view broadly fusiform, about twice as long as broad, the body 
bearing on each side, near the acutely cuspidate extremities, 
two short acute divergent mucrones. Zygospore unknown. 
Measurements.—Length of frond, +3;;; breadth, z5; 
at constriction, =;';5; depth (thickness), =, of an inch, 
