PERIODICITY OF THE PHYTOPLANKTON OF SOME BRITISH LAKES. 481 
and the cells are proportionately longer. Moreover, although this Desmid is 
common in the plankton of the British lakes, typical S. pulchrum does not 
exist in any part of the British Islands, or even in Europe. 
3. ANKISTRODESMUS TORTILIS, W. & G. S. West. (РІ. 19. figs. 9, 10.) 
Cellulæ in coloniis parvis tortilibus subelongatis sublaxe aggregate, inter 
algas limneticas repertee, singulae minute, angustissimæ et acutissim:e, 
spiraliter contortee, anfractee 1-11; chromatophora pallide viridia sine 
pyrenoidiis. 
Long. cell. 12-18 ш; lat. cell. 0°3-0°5 д. 
Наб. Ennerdale Water, Cumberland (May and Aug. 1909). 
This minute species is characterised by the spirally twisted colonies, 
reminding one to some extent of a bit of yarn or a wisp of wool. The cells 
are very minute, with a pale green chloroplast, and are spirally twisted to 
the extent of one, or rather more than one, complete spiral. Each individual 
cell when isolated is very like a miniature representation of the solitary 
A. falcatus var. spirilliformis. 
4. ТЕТВАЗРОВА LIMNETICA, W. & G. S. West. (РІ. 19. fig. 13.) 
Colonize parvee, globose, subglobosze vel indistincte lobatze, libere natantes 
inter alias algas limneticas ; cellule рагу, globosee, subconferte, indistincte 
2-—4-aggregatz ; pseudocilia distincta et breva. 
Diam. colon. 124-220 ш; diam. cell. 4 р. 
Hab. Ennerdale Water, Cumberland (Aug. 1908). 
This tiny limnetie species seems quite unlike any other, and can be 
compared only with Т. lacustris, Lemm., from which it differs in its more 
rounded colonies, with much more numerous and smaller cells, and in its 
distinct pseudocilia. 
5. APHANOCAPSA DELICATISSIMA, W. & G. S. West. (РІ. 19. figs. 2, 3.) 
(Colonize parvee, subellipsoideze, libere natantes, tegumento firmo gelatinoso 
ochraceo et hyalino ; cellula minutissima sphærica solitaria geminatave, 
subconferta ; cytioplasma homogeneum et læte zeruginosum. 
Long. colon. 18-32 р, lat. 14-23 ш; diam. cell. 0°5 д. 
Hab. Wastwater, Cumberland (Jan. 1910). 
This minute species is characterised by Из free-floating habit and 
the minuteness of its cells. It should be compared with A. elachista, 
W. & С. S. West, from which it differs in the cells being only one-third 
the size, and in the more crowded colony. 
6. APHANOCAPSA ELACHISTA, W. & G. S. West, “Some Freshw. Alg. W. 
Indies," in Journ. Linn. Soc., Bot. xxx. 1894, p. 276, t. 15. ff. 9, 10. 
