Protococcacece 217 



Neagh, Ireland; diam. cells 4-8 9'5 /x; diam. ccenob. of 4 cells 10'5 1 

 fig. 90 F. All the species are regular constituents of the freshwater plankton. 

 C. irregularis Wille 1 is known from the plankton of several lochs in the 

 Shetlands, and from Norway. It is a most interesting species, differing from 

 C. rectangularis only in the somewhat irregular colonies and the absence of 

 pyrenoids from all the cells of the colony. Some of the cells in a colony of 

 C. rectangularis are often destitute of pyrenoids, and Wille was quite right 

 in placing his Norwegian plant (C. irregularis) as a species of Crucigenia. 

 Unfortunately, however, the useless generic name ' Willea' has been put for- 

 ward by Schmidle 2 to include Crucigenia irregularis. This genus is simply 

 founded upon the absence of pyrenoids from all the cells of the colony ; and 

 Lemmermann 3 has since placed ' Willea' as a subgenus of Cohniella, also 

 owing to the absence of pyrenoids ! Surely, in Willea, Lemmermannia, and 

 Lemmermann's suggestion that Willea should be a subgenus of Cohniella, the 

 climax of absurdity has been reached with regard to the presence or absence 

 of pyrenoids as a generic distinction. No one who has been fortunate enough 

 to observe colonies of Crucigenia irregularis could dispute their close affinity 

 with forms of C. rectangularis. The former species has most likely had a 

 direct origin from the latter. 



Genus Tetrastrum Chodat, 1895. [Cohniella Schroder, 1897.] 

 The coenobium consists of four cells arranged in one plane and 

 surrounded by a thin mucous envelope. The cells are rounded or 

 somewhat angular, and they possess from two to five spines of 

 variable length on their external margins. The multiplication is 

 by the formation of autocolonies of four cells in each mother-cell. 

 Pyrenoids may be present or absent. The genus only differs from 

 Crucigenia in the presence of the marginal spines and in the 

 regular 4-celled condition of the coenobium. 



The only two known species of the genus are T. heteracanthum (Nordst.) 

 Chod. and T. Staurogeniceforme (Schrb'd.) Chod. The former is known from 

 West Ireland and the latter (fig. 90 G and H ; diam. of cells without spines 

 3 6 p.) occurs in North Yorkshire. 



Sub-family III. SELENASTRE.E. 



This group of the ProtococcaceaB is characterized by the elon- 

 gation of the cells, which are often very narrow with the extremities 

 attenuated to fine points. They are frequently lunate or arcuate, 

 and may be solitary or associated to form colonies of a more or less 



1 Wille, 'Algologische Notizen IV,' Nyt Magazin for Naturvidenskb. Bd 38, 

 Heft 1, 1900, p. 10, t. 1, f. 15. 



2 Schmidle, 1. c. p. 157. 



3 Lemmermann in Berichte Deutsch. Bot. Gesellsch. 1904, Bd xxii, p. 22. 



