80 DR. С. S, WEST ON THE ALGÆ OF 
I have only observed five specimens of this interesting Gyrosigma, and if 
the examination of further individuals indicates that its characters are constant, 
it would perhaps be better to regard it as a new freshwater species. Ш Cleve's 
separation of the genera Gyrosigma and Pleurosigma be accepted (cf. Cleve, 
Synops. Navie. Diat. i. (1894) pp. 32 and 112), the Australian species wouid 
come under the latter genus ; if his separation be not accepted, then it is a 
species of Gyrosigma. 
Class PERIDINEZE. 
Order PERIDINIALES. 
Family PERIDINIACEJE. 
Genus PERIDINIUM, Ehrenb. 
66. Peripinium Уошп, Lemm. in Abh. Nat. Ver. Brem. xvii. (1904) 
р. 166, t. 11. figs. 15-18. 
Var. AUSTRALE, var. п. (Text-fig. 10, A-G.) 
Var. paullo major: hypovalva eum tabulis antapicalibus duoi us asym- 
metricis ; tabulis rhomboideis et tabulis apicalibus 4 epivalvee cum iis forme 
typicæ non plane congruentibus : fossa longitudinis ut visa ventrali sine 
spira minuta terminali. 
Long. 40-56 д; lat. 39-59 д. 
Hab, Yan Yean Reservoir, in the plankton (mostly from Nov. 1905 to 
Feb. 1906) and at the weedy margins (May and Dec. 1905). 
The general disposition of the plates composing the cell-wall is very similar 
to the arrangement in a species of Peridinium recently found at Singapore, 
and described by Lemmermann as P. Volzit. There are, however, certain 
differences in the shape of the four apical plates and the two dorsal plates, 
and in the asymmetrical disposition of the two antapical plates, not to 
mention a few other minor distinctions. 
Mr. Lemmermann very kindly examined both specimens and drawings of 
the Australian form, and as he regards it as somewhat different from the 
organism he recorded from Singapore, I have described it as a variety of that 
species. 
The most interesting feature exhibited by P. Vols var. australe is the 
asymmetrical disposition of the two antapical plates, as such a character 
leads one to suspect that the Australian form is one of the connecting links 
between the two genera Peridinium and Gonyaulax. In the latter genus the 
hypovalve possesses one central antapical plate and an occasional plate, and it 
is quite conceivable that by a further slight rotation of the two antapical 
plates of Р. Volzü var. australe, until one of them attained a central position, 
