VARIATION IN THE DESMIDIER. 413 
Staurastrum Dickiei, &c.; so much so that Meneghini *, and 
afterwards Jacobsen T, were induced to regard Arthrodesmus Incus 
as a species of Staurastrum, and Nordstedt and Lofgren t were 
undecided under which genus to place Arthrodesmus psilosporus. 
The main line of descent from Xanthidium to Arthrodesmus is 
conspicuously evident by the occurrence of species which have 
been referred, with almost equal rectitude, to each of these 
genera $; and it may be well here to emphasize the fact that 
although species belonging to either of these genera are as a 
rule easily distinguishable, yet the only valid difference between 
the two genera is the presence of the central protuberance in 
Xanthidium || and its entire absence in Arthrodesmus, 
The genus Onychonema is a natural derivative of Arthro- 
desmus, by the development of the apical processes and consequent 
assumption of a filamentous condition, and a slight reduction of 
these apical processes would result in the formation of the genus 
Spherozosma, the connecting processes of S. Aubertianum var. 
Archerii being of exactly the same nature as those of an Onycho- 
nema. In the evolution of the genus Streptonema the apical 
processes were further specialized, and a modification of these 
structures to form broader projections of the apical part of the 
cell-wall resulted in the production of those species now placed 
under the genus Desmidium. This view I have further confirmed 
by the examination of a species of Desmidium from Ceylon, 
exactly intermediate between this genus and Streptonema. In 
some species of Desmidium the broad apical processes are very 
* Meneghini, ** Synops. Desm.,” Linnia, 1840, p. 228. 
t Jacobsen, “ Desm. Danm.,” Botanisk Tidsskrift, vol. viii. 1874, p. 204. 
t Nordstedt & Lofgren in Wittr. & Nordst. Alg. Exsic, 1883, No. 558, c. fig, 
$ Xanthidium tetracentrotum, Wolle, Desm. U.S. p. 95, t. 22. ff. 8-9, and 
Arthrodesmus incrassatus, Lagerh. in Öfvers. af K. Sv. Vet.-Akad. Förh. 1885, 
no. 7, p. 242, t. 1. f. 18 ; cfr. West & G. S. West in Trans. Linn. Soc., Bot. ser. II, 
vol. v. p. 253, t. 15. f. 24. Also Xanthidium acanthophorum, Nordst. in Acta 
Univers. Lund. vol. xvi. 1880, p. 11, t. 1. f. 20; referred by Raciborski to Arthro- 
desmus in Pamietnik Wydz. iii. Akad. Umiej. w Krakowie, tom. xvii. 1889, p. 97, 
t. 6. f. 18. 
| There are, however, a few species of Aanthidium without this central pro- 
tuberance (X. leiodermum, Roy. & Biss., X. bengalicum, W. B. Turn.), and many 
others in which it is only slightly developed. 
«| Cfr. West & G. S. West in Trans. Linn. Soc., Bot. ser. II. vol. v. p. 230, 
t. 12. ff. 7-8; Gutwinski in Sprawozd. Kom, fizyj. Akad. Umiej. w Krakowie, 
tom. xxvii. p. 29, t. 1. f. 4. 
