VARIATION IN THE DESMIDIER. 411 
fact that species of both these genera are often found associated 
together amongst mosses on wet rocks, or in Sphagnum-bogs, 
and that the Mesotenia are the more predominant, it seems highly 
probable that at least one series of species of Spirotenia were 
derived from Mesotenium. 
From Penium there are two offshoots, one terminating in 
Roya and the other passing through Closterium to Pleurotenium 
and Docidium; but the main line of descent passes on direct 
from Penium to Tetmemorus, the latter genus ultimately giving 
rise to Zuastrum through such forms as Tetmemorus fissus * 
and Euastrum giganteum t. From Tetmemorus we get an offshoot 
of two rare genera, first Ichthyocereus, and from it, Triploceras. 
With regard to the large genus Cosmarium, embracing as it 
does a multitude of forms, I think there can be little doubt that 
it originated from Ewastrum, many species being met with 
intermediate between these two generat; and that the genus 
Micrasterias was similarly derived from Ewastrum is clearly indi- 
cated by the presence of such species as Mierasterias Moebii and 
M. euastroides $. The genus Cosmocladium has unquestionably 
arisen as a solitary offshoot from Cosmarium; and it is highly 
probable that another small offshoot accounts for the genera 
Spondylosium and Phymatodocis, the main liue of descent passing 
along two courses, one to the genus Xanthidium and the other 
to the genus Staurastrum. 
The genus Xanthidium has been derived chiefly from Cosmarium 
and probably in part from JMierasterias. Some forms of the 
species Xanthidium concinnum and X. Robinsonianum are very 
closely allied to Cosmarium Heimerlii and C. sphagnicolum, and 
* Tetmemorus fissus, West & G. S. West, “Welw. Afric. Alg.," Journ. Bot. 
1807, p. 81, t. 368. f. 25. 
+ Euastrum giganteum, Nordst. in De Toni, Syll. Algar. p. 1106 (1889) ; 
Tetmemorus giganteus, Wood, in Proc. Acad. Nat. Sci. Philad. 1870, p. 19. 
t Cfr. Euastrum cosmaroides, West & G. S. West, " Alg. Madag.," Trans. Linn. 
Soc. ser. II. Bot. vol. v. p. 54, t. 6. f. 28; E. subornatum, West & G. S. West, 
* N.-Amer. Desm.” Lo vol. v. p. 245, t. 14. f. 40; Cosmarium sublobatum, 
Arch. in Pritch. Infus. ed. 4, 1861, p. 731 (= Euastrum sublobatum, Bréb. in Ralfs 
Brit. Deem. 1848, p. 91, t. 32. f. 4); C. subbinale, Lagerh., “ Chloroph. aus 
Abessin. u. Kordofan,” Nuova Notarisia, ser. iv. p. 164; C. genuosum, Nordst., 
“ Algolog. smásak.," Botan. Notiser, 1887, p. 161. 
§ Micrasterias Mocbii, West & G. S. West, “ Desm. Singapore," Journ. Linn 
Soc., Bot. vol. xxxiii. 1807, p. 162; M. ewastroides, Josh., “ Burm. Desm.,” 
Journ. Linn. Soc., Bot. vol. xxi. 1886, p. 637, t. 22. f. 14. 
LINN. JOURN.— BOTANY, VOL. XXXIV. 2r 
