32 COCCOPHYCEE. 
Polyedrium tetraedricum. dg. Finz. Alg. 
Cells regularly tetrahedrical; angles obtuse, mucronate. 
Size. Cells 015-03 mm. diam. 
Rabh. Alg.iii. 62. Archer, Micr. Journ., 1866, p. 62. 
In pools. 
This might possibly be mistaken for the end view of some species of 
Staurastrum, against which error it is essential that beginners should be 
cautioned. 
Plate XI1I. fig. 3. Cells magnified 400 diameters. 
B. Angles radiato-elongated. 
Polyedrium longispinum. (Perty.) Rabh. dig. 111, 62. 
Quadri-radiate, radii thin, elongated, scarcely thickened into 
a body in the centre. 
Size. Length of arms :03--05 mm. 
Phycastrum longispinum, Perty Kl. Lebensf. t. xvi., f. 30. 
In pools. N. Wales (A. W. Wills). 
A peculiar species, which at first sight seems to have but little re- 
lationship with the other species figured. It is often found associated 
with Desmids, and delights in similar localities. 
Plate XIII. fig. 2. a, b, ve, d, cells magnified 400 diameters. 
C. Angles lobed. 
Polyedrium enorme. (Raljs.) Rabh. Alg. 111., 63. 
Cells irregularly tetrahedrical, with the angles produced, 
hyaline, deeply bilobate ; sometimes repeatedly bilobed, with the 
lobes mucronate. 
Size. Cells :025-:04 mm. diam. 
Staurastrum enorme, Ralfs, t. 33, f. 11. 
In pools. 
“Frond very irregular and variable in form. Sometimes the front 
view differs but little from the end one. Usually, however, there is a 
slight constriction at the junction of the segments, but I have never ob- 
served any difference in the endochrome at that part. The spines, which 
are almost confined to the angles, are irregular, some simple and some 
branched. The end view has three or four broad and very irregular 
lobes; these are spinous and more or less emarginate, and frequently 
one lobe is much broader and more spinous than the others. The spines 
on each lobe form two groups, separated by the notch; they vary much 
in size, and are either simple and subulate, or else forked; sometimes 
the forked spines are again divided at the apex.”—Ralfs. 
A very variable plant, formerly included with the Desmidiex, but 
separated on account of its different mode of propagation. 
Plate XIII. fig. 4. Cells magnified 400 diameters. Lateral and end 
views. 
Sub-Family 4, ScENEDESMES. 
Cells elliptic, oblong, or cylindrical ; cell-membrane very thin ; 
cell-contents at first homogeneous, afterwards granular ; chloro- 
phyllose vesicle central or sublateral, and often a lateral eolour- 
less spot; cells 2-4-16, either joined in a single series or forming 
