VARIATION IN THE DESMIDIER. 393 
two series, the latter being most obvious in slightly tilted speci- 
mens (cfr. Pl. 11. figs. 17 b” and 196). Typically there is but 
one series of four spines, the two middle spines being the largest. 
In some cases, however, the median pair alone are present; and 
in other specimens there may be a second series of four smaller 
spines beneath the larger series. In vertical view, the lateral 
margins may be concave, straight, or slightly convex, all these 
differences being frequently met with in the same gathering. 
The spines present at the lateral angles may be irregularly dis- 
posed or may be arranged one above another in a vertical plane. 
It is a small species aud retains a relatively constant size: 
long. sine spin. 21-25 p ; lat. sine spiu. 19-26 a; lat. isthm. 6:5— 
15 p. 
15. STAURASTRUM CRENULATUM, Delp., in Mem. R. Accad. Sci. 
Torino, ser. ii. tom. xxviii. 1877, p. 68, t. 12. ff. 1-11. Phycastrum 
erenulatum, Naeg. Gatt. emz. Alg., Zürich, 1849, p. 129, t. 8. B.— 
Much variation was observed in a large gathering of this species 
from Roundhay Park, W. Yorks. Some specimens were con- 
siderably broader than long (including the processes), and these 
usually possessed the emarginate warts developed at their 
maximum ; long. 22-23 u; lat. e. proe. 27-33 u. A series of 
forms were observed which exhibited a gradual shortening of the 
processes, and consequently a proportionate increase in the 
length of the cells: long. 23 a; lat. c. proc. 21-24 u. Those 
forms with very short processes were sometimes remarkable for 
the reduction of the emarginate warts on the apices of the semi- 
cells (cfr. Pl. 11. figs. 21-27). Some examples possessed much 
in common with S. margaritaceum, Menegh., var. ornatum, 
Boldt (in Ofvers. af K. Vet.-Akad. Fórh. 1885, no. 2, p- 116, 
t. 5. f. 27 ; since placed by Turner as a species—S. ornatum). 
16. STAURASTRUM ACULEATUM, Jenegh., “ Synops. Desm.,” 
Linnea, 1840, p. 226.— This plant, first described by Ehrenberg 
(Die Infus. als vollk. Organ., Leipzig, 1838, p. 142, t. 10. f. 12) 
as Desmidium aculeatum, 1s one of the most characteristie species 
of the genus. Yet it is one whieh has been frequently mis- 
understood, large numbers of forms having been figured under 
the name of S. aculeatum which belong to widely different 
species. Figs. 28-32, Pl. ll. are accurate representations of 
both European and American forms. As indicated by its spe- 
cifie name, the spines are well developed and are always arranged 
