390 MR. 6. 8. WEST ON 
scarcely appreciable swelling on each side to a protuberance of 
some magnitude (cfr. Pl. 10. figs. 27, 28). In 1879 Wille de- 
scribed a var. intermedium of this species (“ Ferskv. Alg. fra 
Nov. Seml.,” Ofvers. af K. Vet.-Akad. Fürh. 1879, no. 5, p. 35, 
t. 12. f. 15), and in 1888 Boldt described two torms—forma 
groenlandica and forma subconspersa (“ Desmid. från Grénl.,” 
Bihang till K. Sv. Vet.-Akad. Handl. Bd. xiii. Afd. iii. no. 5, 
p. 25, t. 2. f. 26); but these forms do not possess any characters 
sufficiently definite for thera to have received varietal names, as 
all three of them, and all intermediate stages, were abundant in 
the gathering from Welsh Harp, Middlesex. Thus it may be said 
that the semicells of typical Cosmarium biretum may possess any 
approximately subrectangular or polygonal contour, and in 
vertical view may possess anything between a well-marked pro- 
tuberance in the middle on each side and a scarcely appreciable 
inflation. 
A second type of this species, more rarely found, exhibits 
rather different characters. In 1875 Nordstedt described two 
forms (* Desm. Arct.,” Öfvers. af K. Vet.-Akad. Förh. no. 6, 
p- 26, t. 7. ff. 18, 19) which he named forma supernumeraria 
and subsp. trigibberum. These are each characterized by the 
possession of three protuberances on each side in the vertical 
view, a central one and one close to each end. I always find 
this second type of C. biretum, although showing a considerable 
range of variation in outline, to be more rounded in general 
— | 
Proportion of | 
breadth to 
length Measurements 
Measurements 
Cosmarium biretum. | (lat. : long.). of largest | of smallest Average 
LÀ : thickness. 
verage of a form. form, 
number of 
| specimens. 
First type: with one | Long. 48 p; lat. | “Tong, 38 p; lat. 
inflation on each | E | Qn. 9 
. . : 1:113 BEI p: lat.) 35g; lat. 23 p. 
side ol vertical : = < . 
view | isthin. 15 a. isthm. 13 a. 
Second type (var. | | | 
trigibberum): with | Long. 57°5 p; | Long. 43 u; lat. 
three ` inflations | 1: 1-04 lat. 54 u; lat.) 38 p; lat. 32 p. 
on each side of isthm. 21 p. | isthm. 12 u 
vertical view. | | | 
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—j— mE — 
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