ARCHER, ON TWO NEW SPECIES OF COSMARIUM. 179 
what longer than broad, broadly elliptic, ends rounded ; end 
view broadly elliptic; empty frond colourless, not punctate. 
Zygospore unknown. 
Measurements. — Length, ;1,; breadth, -'>;; depth, 
ss's@ Of an inch. 
Fig. 32, front view; fig. 33, side view. 
Affinities and differences.—The oblong non-crenate figure 
and smooth surface of this little form will readily distin- 
guish it from every other at all agreeing with it in dimen- 
sions, such as Cosmarium Meneghinii ; its size is, besides, 
in every way maller than that of that species. It is, indeed, 
amongst the largest species that a similarity of figure is to be 
found ; and indeed, as far as concerns outward form alone, 
it is difficult to define in a diagnosis the characters which 
separate this little species from C. cucumis, Corda. In both 
the frond is oblong and smooth, deeply constricted ; the seg- 
ments subquadrate. But the linear dimensions of this new 
form are some three or four times less than those of the 
latter ; moreover, the former is sometimes more than twice 
as long as broad—the latter is less than twice as long as 
broad; and, leaving the dimensions out of view, this cha- 
racter, apparently slight, would help to an identification. 
But as concerns dimensions, it would be as little necessary to 
compare Docidium minutum with D. nodulusum or D. trun- 
catum, or Euastrum elegans with E. oblongum or E. crassum, 
or Closterium Cornu with C. acerosum, &c. This new form 
differs, too, almost as greatly in size from Cosmariwm quad- 
ratum, Ralfs, and moreover wants the protuberance at each 
side near the base of each segment present in that species. 
Thus, though the agreement in figure of this new form with 
the species referred to it is considerable, I cannot fancy 
their beg mistaken. But, moreover, the endochrome in 
this new form has embedded in it in each segment but one 
central large starch-granule. From C. cucurbita, Bréb., 
this form is quite distinguished by its deep linear constric- 
tion and non-punctate cell-wall, besides dimensions and 
other special points at once recognisable by those acquainted 
with these species ; and, besides those mentioned, there are 
none others with which it is in the least necessary to be con- 
trasted. 
Genus—Perntum, Bréb. 
Prentum MoorEANnvM, sp. noy. 
Specific characters,—Frond very minute, about one third 
longer than broad, sides somewhat barrel-shaped, ends trun- 
