198 BRITISH DESMIDIACEJE. 



less spherical or sometimes angular, with short stout 

 spines or obtuse protuberances. 



The genus Cosmodadium is closely related to Cosmarium, from 

 which, undoubtedly, it was originally derived. Its chief distinc- 

 tion from that genus is that the cells are joined together by means 

 of slender gelatinous stalks, which are attached in the vicinity 

 of the sinus. In this wav, more or less branched colonies of 



/ 



varying size and shape are formed. It has been shown by Dr. 

 Lutkemuller that the connecting gelatinous threads are secreted 

 by special groups of pores situated near the base of the semicell.. 

 There may be single strands, or sometimes a pair of parallel 

 gelatinous filaments joins the cells, depending on the number 

 of pore groups present. In C. constrictum each semicell possesses 

 one series of pores at its base, and here a single strand connects 

 the cells, but in C. Saxonicum there is a group of these special 

 pores on each side of the isthmus in each semicell, and in conse- 

 quence the connecting strands are paired. In some of the 

 other species it is impossible to say definitely whether the con- 

 necting strands are single or double, since they have not been 

 sufficiently investigated on this point, and because the strands 

 are so delicate that it is not easy to decide the question. The 

 entire colony is, in addition, sometimes immersed in a delicate 

 mass of jelly. 



The genus Cosmodadium was much confused in earlier times 

 with the Protococcales genus Dictyosphcerium. It is most 

 readily distinguished from this genus, however, not only by its 

 constricted cells, but also by the fact that the cells are placed 

 at various points, not merely at the periphery of the colony. 



There are 5 British species of the genus, all of which are very 

 rare. They are very minute and inconspicuous, and are very 

 easily overlooked. 



1. Cosmodadium constrictum Arch. 

 (PI. CLVIII, figs. 1-3.) 



Dictyosphcerium sp. Arch, in Quart. Journ. Micr. Sci. 1865, p. 127. 



D. constrictum Arch. ibid. 1867, p. 299 : ibid. 1872, p. 422. 



Cosmodadium constrictum Arch. ibid. 1875, p. 415 ; Josh, in Journ. Bot. 

 1883, p. 292 ; Cooke, Brit. Desm. 1887, p. 79 ; De Toni, Syll. Alg. 1889, 

 p. 805 ; Roy & Biss. Scott. Desm. 1893. p. 254, t. 2, f. 7 ; Liitkem. Zell- 

 membr. Desm. 1902, p. 359, t. 18, f. 31-33 : West & G. 8. West, Alga-fl. 

 Yorks. 1902, p. 95 ; G. S. West, Brit, Fresrtnv. Alg. 1904, p. 173, f. 66 A. 



Cells very small, rather more than 1| times longer 



