176 ARCHER, ON TWO NEW SPECIES OF COSMARIUM. 
Plate VI, fig. 50, front view; 51, side view; 52, end view 
of frond; 58, 54, dividing fronds; 55, abnormal frond. 
Affinities and differences—As to the specific distinctness 
of this minute form, there seems to me not the smallest doubt 
or difficulty. There might, however, in the opinion of some, 
be a question as to its generic position ; for it seems possible 
that the same conflicting views which have been held by 
different observers as regards Arthrodesmus octocornis might 
also be held with respect to this new form. <Arthrodesmus 
octocornis, Ehr., Hass., Bréb., &c., was at one time placed in 
the genus of Xanthidium and Ehrenberg—Meneghini and 
Kiitzing considered it a Micrasterias—Ralfs looks upon it as 
doubtfully a Xanthidium—but whatever be the proper position 
of that species, there can be little doubt, I think, but that the 
present form belongs to the same genus, and I should un- 
doubtedly be disposed to consider that to be Arthrodesmus. 
Indeed, the only question, as it appears to me, is whether it 
be Xanthidium or Arthrodesmus. ‘Taking Arthrodesmus incus 
or A. convergens as typical of the genus, there does, indeed, 
appear some dissimilarity between them and such as A. bifidus, 
Bréb., A. octocornis, and the present form, but I imagine 
the former are more closely related to certain Staurastra than 
are the latter to Xanthidium. For A. convergens and A. 
octocornis may, lL imagine, be almost looked on as Staurastra, 
two-sided, not three—or more—sided, in end view—the first 
having most affinity to such forms as Staurastrum Dickiet, or 
S. dejectum, or S. brevispina, the latter having greatest rela- 
tionship to S. glabrum or S. O?Mearui. Three or more im 
number of sides (or angles) in end view, in Staurastrum, is 
quite well known not to be of any generic or even specific 
value, therefore two only may be of as little import. But m 
giving expression to such a view, whatever might be the dif- 
‘ference of opinion as to the generic position in the case of the 
species just mentioned, there cannot, I should think, be the 
smallest donbt as to their specific distinctness; but that is 
not the question at present. Arthrodesmus octocornis, on the 
other hand, seems to possess (especially through a plant I 
myself described as Xanthidium Smithii) considerable affinity 
to Xanthidium, in which genus it was hesitatingly placed by 
Ralfs. But it differs therefrom in having its marginal spines 
disposed in a single, not a double, series, nor scattered; and 
secondly, and, as [ think, in a more important circumstance, 
in its wanting the prominences occupying the centre of both 
front surfaces of each segment characteristic of Xanthidium. 
Again, Arthrodesmus bifidus, Bréb., cannot at all be said to 
be spinous, but its sub-reniform quadrangular segments are 
