178 ARCHER, ON TWO NEW SPECIES OF COSMARIUM. 
of the free species of the family, for the concavities of the 
form under consideration render its actual bulk or volume 
very appreciably less. It is, therefore, considerably smaller 
than any other Arthrodesmus. From <Arthrodesmus bifidus, 
Bréb., its straight, not curved and bicuspidate, segments and 
its four terminal mucrones in end view, abundantly distinguish 
it. From <dArthrodesmus octocornis it is distinguished by its 
much less elongate and less acute lateral extremities, which 
in the latter are, as well as the upper angles, prolonged into 
single or double spines, all divergent in front view—while in 
the new form the four external spines are well seen only in 
the edge views, and are not in a single series. From A. 
octocornis, var. [3, as well as from Xanthidium Smithii, (mihi) 
this species is distinguished by its not having a double spine at 
each angle, and from the latter also, by the want of the central 
protuberances. But, besides these more tangible and promi- 
nent diagnostic distinctions, I need hardly remind those who 
are thoroughly acquainted with this elegant family of Alge, 
that each of them—these new ones included—presents to the 
familiarised a special tout ensemble of its own, difficult, per- 
haps, to define in words, but, in my mind, not the less actual, 
constant, distinctive, and characteristic. 
Note.—I have in the foregoing made use of, and shall for 
the future employ, the more apt term “ Zygospore,” as sug- 
gested by Professor De Bary in his work ‘“ Untersuchungen 
tiber die Familie der Conjugaten,” in preference to the seem- 
ingly inappropriate and inaccurate term “ Sporangium” of 
most other authors. 
Genus—Cosmarivum, Corda. 
Cosmarium exiguum, sp. nov. 
Specific characters.—Frond very minute, smooth, oblong, 
rather more than twice as long as broad; segments sub- 
quadrate ; starch granule single, central. 
Locality.x—Dublin mountains ; not very rare. 
General Description.—Frond very minute, smooth; in front 
view, rather more than twice as long as broad, constriction 
deep, linear, acute ; segments in front view very slightly longer 
than broad, sub-quadrate, sides sometimes very slightly 
tapering, angles somewhat rounded, ends rotundato-truncate, 
with a very gentle, often nearly imperceptible, minute central 
depression; starch granule single, central; in side view the 
constriction acute within, widening outwards, segments some 
