VARIATION IN THE DESMIDIEE, 407 
Another precisely similar case is brought forward in studying 
the forms allied to Micrasterias oscitans, Ralts, and M. pinnati- 
Jida, Ralfs. In the first place, what is Micrasterias oscitans, 
Ralfs? It was described by Ralfs * in 1845 and figured by him t 
in 1848. Since that date no one has reported the occurrence of 
the typical form exactly as he figured it, although many forms 
have been described and figured intermediate between M. oscitans 
and a much more abundaut species found some years later (1859), 
viz. M. mucronata, Rabenh. (first described as Tetrachastrum 
mucronatum, Dixon f) M. mucronata is a species frequently 
met with in upland Sphagnum-bogs, and the plant described as 
M. oscitans by Ralfs is unquestionably an extreme form of the 
same species, and, as it seems, a very rare one also. It is un- 
fortunate that the form first found and described was the rare 
and aberrant one, as its specific name must be used, whereas the 
form commonly met with has to stand as a variety of it (viz. 
JM. oscitans, Ralfs, var. mucronata, Wille S). There can be no 
doubt that both M. oscitans, Ralfs, and M. laticeps, Nordst. ||, 
have arisen from that assemblage of forms known under the 
name of M. oscitans var. mucronata, but the origin of the latter 
is somewhat doubtful. After much consideration I should be 
inclined to think that the line of descent came through M. depau- 
perata, Nordst. f, a decided relationship being evident between 
certain forms of M. oscitans var. mucronata, such as those 
described by Cooke **, and some forms of M. depauperata tt. 
In temperate regions Micrasterias pinnatifida is a species of 
greater rarity than M. oscitans var. mucronata. It was first 
described by Kuetzing ii as Euastrum pinnatifidum, and many 
authors have followed Rabenhorst $$ in placing it as a variety of 
* Ralfs in Jenner, Fl. Tunbridge Wells, 1845, p. 198. 
+ Ralfs, Brit. Desm. t. 10. f. 2. 
+ Dixon in Trans. Dubl. Nat. Hist. Soc. 1859, p. 204, t. 1. ff. 5-8. 
$ N. Wille, “ Bidrag till Kundsk. om Norges Ferskv.-Alg.," Christiania Vid.- 
Selsk. Forhandl. 1880, no. 11, p. 21, t. 1. f. 3. 
|| Nordstedt in Vidensk. Medd. f. d. nat, Foren. Kjöbenhavn, 1869, p. 220, 
t. 2. f. 14. 
«| Nordstedt, 7. c. p. 222, cum fig. xylogr. 
** Cooke in ‘ Grevillea,’ vol. ix. 1881, p. 89, t. 141. ff. 25, c. 
tt Gf. the form in West & G. S. West, ** Some N.-Ame:. Desm.,” Trans. Linn. 
Soc., Bot. ser. IT. vol. v. pt. 5, p. 238, t. 14. f. 1. 
tt Kuetzing, Phycol. German. 1845, p. 134. 
$8 Rabenhorst, Krypt. Flor. Sachs. 1862, p. 184; Flor. Europ. Algar. iii. p. 189. 
