STAURASTRUM. 121 



between this species and St. bibrachiatiim Reinscli var. cymatium 

 AVest (= : Dichotomum bibrachiatiim W. & G. S. West var. cyma- 

 tium). A reduced form of the latter species has also been 

 described and figured by West & G. S. West ('Alg. Madag.' 1895, 

 p. 74, t. 8, f. 28 a' and b'). Bohlin criticises the creation of the 

 new genus Dichotomum in view of the discovery of such forms 

 as these, which obviously link it up with Staurastrum. It must 

 be pointed out, however, that practically all divisions between 

 allied genera are more or less arbitrary, and that as a rule no 

 hard and fast line can be drawn between them. This is particu- 

 larly true in the case of the Desmidiacea3, and species forming 

 links between other genera of the group have already been noted 

 in this work (see vol. ii, p. 126 ; vol. iv, p. 89). Nevertheless 

 no one would desire, for example, that the genus Arihrodesmus 

 be abolished because it is closely linked up with some forms of 

 Staurastrum. In the same way it will probably be very con- 

 venient to retain the genus Dichotomum also, although it is very 

 closelv allied to Staurastrum. 



/ 



Var. validum W. & G. S. West. (PL CXLIX, fig. 5.) 



Xt. tetmcerum var. ralidum West & G. 8. \Yest, Alg. S. England, 1897, 

 p. 495, t. G, f. 25. 



A rather larger form, with the body of the cell rela- 

 tively longer ; processes stouter and not at all attenuated, 

 with 5 undulations. 



Length, without processes. 18 [/. ; with processes, 42 u : 

 breadth, without processes, 13 UL ; with processes, 

 breadth of isthmus 5 (JL . 



ENGLAND. Near Chapel Wood, Surrey ! 



125. Staurastrum iotanum Wolle. 

 (PL CXLIX, fig. i.) 



.in iotanum Wolle, Desm. U. S. 1884, p. 137, t. 51, f. 5-7; De Toni, 

 Syll. Alg. 1889, p. 1147 ; West, Alg. N. Wales, 1890, p. 20 ; Roy & Biss. 

 Scott. Desm. 1893, p. 21 ; Turn. Alg. E. India, 1893, p. 132, t. 22, f. 12 ; 

 West & G. S. West, Alg. S. England, 1897, p. 495 ; Desm. U. S. 1898, 

 p. 314, t. 18, f. 14, 15 ; Alg. Ceylon, 1902, p. 185. 



Cells very minute and inconspicuous, slightly broader 

 than long, including the processes, constriction fairly 

 deep, sinus small, acute-angled and open ; semicells 



