1114 Proceedings of Royal Society of Edinburgh. [sess. 
out any figure. Chodat has emended the diagnosis of the species 
(1902, p. 189), but he does not mention the pyrenoids, while 
these are present in West’s drawing. 
Bacillariace^:. 
Asterionella formosa, Hassall. — The Asterionella , which inhabits 
Thing vallavatn, is rather short and robust, thus representing the 
typical A . formosa. 
Fragilaria. — With regard to the species of Fragilaria , my 
determinations may he insufficient ; this genus is a very difficult one. 
No doubt the species are connected with each other without 
distinct limits. The easiest discernible form is Fragilaria 
crotonensis , (Edw.) Kitton, distinguished by the intervals between 
the cells ; it is not common in Thingvallavatn, and occurs only in 
the summer and autumn. The commonest Fragilaria in the 
plankton is a small bottom form, F. construens, (Ehbg.) Grun., of 
which I have given a figure of a filament in my previously quoted 
paper (1904. p 232, fig. 3). Very near to it, and mostly distin- 
guished from it by the denser striae, is F. capucina , Desmaz, 
which also often is present in the samples. 
The last, again, goes over into the F. virescens. It seems to me 
as if the limit between F. construens , F. binodis, (Ehbg.) Grun., and 
F. capucina is not clear, and I fear that my identifications of these 
two species may have been arbitrary. 
Synedra. — Besides S. ulna, (Nitzsch) Ehbg., another or several 
other species occur in the samples ; they are smaller, narrower, and 
slenderer, but they vary much in length and breadth. Most of 
these individuals are 70-150 g long and 2-4*5 g broad ; they agree 
well with the figures by Yan Heurck (1880-1881, pi. xxxix. 
fig. 6) of S. delicatissima, W. Sm., v. mesoleja, Grun., but this form 
is very near to S. radians , W. Sm. (Yan Heurck, l. c. pi. xxxix. 
fig. 11). I take all these species as forms of S. acus, Kiitz., and 
name our form S. acus, f. delicatissima, (W. Sm.) Grun. The 
specimens have in most cases been found one by one, but occasion- 
ally 2, 4, 6 or a still greater number of individuals were arranged 
in a radiate manner, thus forming stellate colonies, probably an 
adaptation for the limnetic life (see PI. II. figs. 16, 17).* Also 
* Cp. the colony of S. radians, by W. Smith, 1853, pi. xi. fig. 89. 
