Protococcacefp 



221 



Lagerh. var. linearis Hansg. (fig. 92 I K) is widely distributed, but rarely 

 occurs in such abundance as the three preceding species. The cells of S. cos- 

 tatus Schmidle and S. acutiformis Schroder possess prominent longitudinal 

 ridges. S. Hystrix Lagerh.' and S. gmnulatus W. & G. S. West are very 

 uncommon species remarkable for their external ornamentation. 



Genus Dimorphococcus A. Br., 1849. The cells are arranged 

 in composite colonies, each colony 

 consisting of an irregular agglome- 

 ration of definite groups of four 

 cells. The cells of each group are 

 disposed obliquely in one plane and 

 are of two kinds; the two central 

 cells are ellipsoid or oblong arid the 

 two outer cells are sublimate. The 

 groups of four are held in position 

 by irregular portions of the old walls 

 of the mother-cells. There is a large 

 parietal chloroplast with one pyre- 

 noid. Autocolonies of four cells are 

 produced in each mother-cell ; these 

 remain attached to the parent-colony 

 until the latter becomes too large, 

 when it breaks up into several 

 smaller colonies. 



Fig. 93. Dimorphococcus IIIIKI- 

 tu$ A. Br. ; A, from the plankton 

 of Loch Mor Bharabhais, Lewis, 

 Outer Hebrides; B, from Bowness, 

 Westmoreland ( x 520). 



D. lunatus A. Br. is a rare plant in 

 Britain 1 . It sometimes occurs in the 

 plankton, but is met with more often in the small tarns of mountainous dis- 

 tricts. The colonies are 57 86 /z in diameter and the cells 11 25 /* in 

 length; fig. 93. 



Genus Ankistrodesmus Corda, 1838 ; Ralfs, 1848 ; Archer, 

 1862. [Rhaphidium Klitz., 1845; Schroderia Lemmermann, 1898.] 

 The cells are fusiform with acute apices, rarely obtuse, and they 

 are generally many times longer than their diameter. They are 

 straight, lunate, arcuate, or sigmoid, and although frequently 

 solitary, are more often variously grouped in loose aggregates. 

 In some forms the apices are greatly produced and almost bristle- 

 like. The cell- wall is very thin and there is a single parietal 

 chloroplast, usually occupying the greater part of the cell-cavity. 



1 Vide West in Journ. Boy. Micr. Soc. 1892, p. 735, t. ix, f. 39; West & G. S. 

 West in Trans. Hoy. Irish Acad. xxxii, sect. B, part i, 1902, p. 66. 



