192 



Mr. W. West and Dr. G. S. West. [Oct. 30, 



Of the colonial unicells, Gcelosphceriwm Kiitzingianum (and the form of it 

 known as C. Ndgelianum), Gomphosphceria lacustris, and Chroococcus 

 limneticus are the most important. Microcystis (Clathrocystis) aeruginosa 

 occurs more particularly in pools and small, shallow lakes, where the 

 temperature of the surface water becomes relatively high in the summer. 



In the Scottish and the Welsh lakes the Blue-green Algae are decidedly 

 scarce. This scarcity is to he attributed to the Alpine character of so many 

 of these lakes, in which the maximum temperature of the surface water is 

 relatively low. 



The Flagellata are well represented by various Peridinieae, by Mallomonas, 

 and by several species of Dinobryon. In many of the larger British lakes 

 Dinobryon completely dominates the spring plankton, and a few colonies 

 generally persist through the summer and the early part of the autumn. 

 The most abundant species is D. cylindricum, and its var. diver gens is equally 

 common. In the smaller lakes Dinobryon is fairly general, but does not 

 attain such great maxima as in the larger lakes. Mallomonas acaroides 

 sometimes occurs in prodigious abundance, but lasts only a few weeks. 



Of the Peridiniea3, Ceratium hirundinclla is general, but it only occurs in 

 large maxima in the smaller lakes. The variations in this organism have 

 been well described and figured,* but there is one form in the lakes of the 

 Outer Hebrides and the west of Ireland f which is apparently unknown in 

 the plankton of the rest of Europe. In this curious form the first antapical 

 horn is very much deflected to one side {vide fig. 3). Species of Peridinium 

 often occur in very large quantity in the smaller lakes, and in the shallower 

 of the large lakes. P. tabulatum is frequent, but P. Willei is general 

 throughout the Scottish, Irish, and English lakes.i It occurs most 

 abundantly in the small upland lakes (up to 1800 feet) with an Alpine 

 character. P. Westii is exclusively confined to the Scottish lakes, where it 

 appears to be frequent. 



The Bacillarie^e are abundant in the British phytoplankton, but they 

 rarely occur in such great quantities as in the Central European lakes, and 

 scarcely ever form the vast maxima which periodically appear in so many of 

 those lakes. Diatoms are fewest in the plankton of the Welsh lakes, 



* Lemmermann, in 'Archiv for Bot. utgifv. af K. Sv. Vet.-Akad.,' Bd. 2, 1904, No. 2, 

 t. 2, f. 1—49 ; W. and G. S. West, in ' Boy. Soc. Edin. Trans.,' vol. 41, 1905, p. 494 (c. fig.) ; 

 'Boy. Irish Acad. Trans.,' vol. 33, sect. B, part 2, 1906, pp. 93, 94 (c. figs. 1—9); 

 Bachmann in 'Archiv fur Hydrobiologie und Blauktonkunde,' Bd. 3, 1907, pp. 55 — 58, 

 figs. I, la, II. 



+ W. and G. S. "West, in 'Boy. Irish Acad. Trans.,' loc. cit., 1906, p. 94, f. 9. 

 \ The distribution of Peridinium Willei, Huitf.-Kaas, extends from N. Italy, Ireland, 

 England, and Scotland to Norway, Finland, the Faeroes, and Iceland. 



