408 MR. G. 8. WEST ON 
Micrasterias oscitans, but for reasons best known to themselves. 
In studying the numerous forms of it and of allied species, one is 
struck by the continuous series they present,—-a series ranging 
from M. Crux-Melitensis, Mass, through M. Rabenhorstii, 
Kirehn., M. pinnatifida var. divisa, West, M. pinnatifida var. 
erpansa, Turner, M. pinnatifida var. inflata, Wolle, M. pinnati- 
fida, Ralfs, M. arcuata var. subpinnatifida, West & G. S. West, 
M. arcuata, Bail., and M. arcuata var. expansa, Nordst. The 
series is complete as we go back through the varieties of M. pin- 
natifida to AL. Itabenhorstii ; and I think there ean be no doubt 
that the latter species is a derivative of JM. Crux-Melitensis, 
the intermediate forms being such as those mentioned by Gut- 
winski *, Raciborski T, and Borge t. A figure of M. Rabenhorstii 
vav. tatrica, also given by Raciborski $, is identical with one 
semicell of the specimen of M. Crux-Melitensis figured by 
Gutwinski. Micrasterias incisa, Bréb., has evidently originated 
from M. pinnatifida var. inflata, Wolle, and M. arcuata var. 
subpinnatifida, West & G. S. West ||, is a form exactly inter- 
mediate between M. pinnatifida and M. arcuata. 
I have already mentioned that many authors have placed 
M. pinnatifida as a variety of M.oseitans, but I strongly disapprove 
this arrangement. It is true that the two species somewhat 
resemble each other in outline, but they differ greatly in size, 
in the form of the lateral lobes, aud especially in the form of the 
polar lobes. From these facts, and from the above-mentioned 
evolutionary series which I have shown to exist, I think that the 
two species can be justly regarded as occupying two extreme 
branches in the phylogeny of the species of Micrasterias— 
branches which have undergone a parallelism of modification 
with regard to external morphological characters, with a result 
that the majority of authors have failed to discriminate between 
two well-marked series of forms. 
* R. Gutwinski, “Flora Glonów okolie Lwowa,” Spraw. Kom. fizyjogr. Akad. 
Umiej. Krakow. tom. xxvii, t. 3. f. 28. 
+ M. Raciborski, * Desm. Polon." Pamietnik Akad. Umiej. w Krakowie, 
Wydz. matem.-przyr. tom. x. 1885, t. 14. f. 4. | 
t O. Borge, “ Sussw.-chlor. Archangel,” Bih. till K. Sv. Vet.-Akad, Handl, 
Bd. xix. Afd. iii. no. 5, t. 3. 1. 40. 
8 M. Raciborski, “ Desmidyje Nowe,” Pamietnik Wydz. iii. Akad, Umiej. 
w Krakowie, tom. xvii, 1889, t. 7. f. 2. 
| West & G. S. West, “ Welw. Afric. Alg.,” Journ. Dot. xxxv. 1£97, p. 86, 
t. 360. f. 7. 
