32 DR. б. $. WEST ON THE ALGÆ OF 
DOMINANT FORMS AND THEIR PERIODICITY. 
Flagellata.—The genus Dinobryon was not so abundant as in the plankton, 
although D. cylindricum var. divergens was common in December. D. ele- 
gantissimum is an interesting stipitate form which was very scarce in 
January. Trachelomonas hispida occurred sparingly several times during 
the year. 
Peridiniem.—T wo species occurred in the warmer part of the year, but 
only one, Р. inconspicuum, was abundant, occurring in quantity in December. 
P. Volzi var. australe was observed very sparingly in both May and 
December, months in which the temperature was approximately the same. 
Chlorophyceæ.—From February to July the Desmids were the dominant 
feature of the benthos, and they attained their maximum abundanee in 
April, with a water-temperature of. 178? С. In August (temp. 10? C.) they 
disappear with remarkable abruptness, the collections for this month con- 
taining thousands of the dead and decaying remains of these plants. Very 
few individuals persisted. until the end of. August, and in September, which 
is the last month of the cold period, Chlorophyceæ of all kinds were entirely 
absent. In October a few Desmids are again noticeable, and during the 
rise of temperature through November and December vast numbers of 
the smaller species make their appearance, amongst which should be specially 
mentioned ` /Zuastrum Turneri forma minor, Cosmarium Capitulum var. 
australe, C. quadratulum, C. pygmeum, С. angulosum, Staurastrum bibra- 
chiatum var. eymatium, ©. tetracerum, and а form of 5. excavatum. 
Although the number of species (67) of Desmids present in the early part of 
the warm season, from November to December, is almost as great as the 
number (81) oceurring during the autumn, from May to July, they do not 
give such a pronounced character to the collections. This is entirely due to 
the dominance of smaller and more inconspicuous species during the vernal 
period, as compared with larger and more ostentatious species which 
predominate in the autumnal period. 
In November, and also to a less extent in December, the Zygnemacez and 
Oedogoniaceæ are dominant, and almost all in the fruiting condition. The 
commencement of the fructiferous state simultaneously with a rising temperature 
is strictly comparable with what occurs in the southern and south-western 
counties of England. l have observed repeatedly in Surrey and Hampshire, 
and more especially has it been brought to my notice in Devonshire and 
Cornwall, that the normal period of fructification in the Zygnemacez and 
the Oedogoniaceæ begins at or towards the end of April, and continues 
through May into June. With regard to thirteen species of the genus 
Spirogyra, Fritsch & Rich * have recently brought forward observations to 
* F. E. Fritsch & F. Rich in Ann, Bot. xxi, (July 1907) pp. 432 & 486. 
