110 BRITISH DESMIDIACE^E. 



Arthrodexmus subulatus is much more frequent in tropical 

 latitudes than in temperate countries. In its typical form it 

 is easily distinguished from A. convergens by the shape of its 

 semicells and the long', horizontally-placed spines. 



Forma americaiia (Turn.) W. & Gr. S. West. (PL 



CXVII, figs. 4, 5.) 



A. Incus var. americanus Turn. New and Rare Desm. 1885, p. 937, t. 16, 

 f. 17 ; De Toni, Syll. Alg. 1889, p. 1057. 



A. triangularis var. americanus (Turn.) West, Alg-. W. Ireland, 1892, 

 p. 169. 



A. elegans West, Alg. W. Ireland, 1892, p. 169, t, 22, f . 7 [gelatinous pro- 

 cesses described as " spinis brevibus delicatis truncato-bifurcatis/'J 



A. subulatus forina media Turn. Freshw. Alg. E. India, 1893, p. 133, 1. 11, 

 f . 36, 37 ; t. 12, f. 4. 



Differs from the type only in the long spines being 

 slightly convergent, and in the semicells being a little 

 less turgid (in outline rather more triangular). 



Length 26-32 ^ ; breadth without spines 22'5-30 /*,, 

 with spines 62-65 //, ; length of spines 16-22 ju, ; breadth 

 of isthmus 7-10 /x; thickness 11-14 ft. 



IRELAND.- -Lakes between Clifden and Koundstone, 

 near Recess, and Ballynahinch, Galway ! 



Geoyr. Distribution.- -India. United States. 



When A. elegans was described from Ballynahinch it was 

 not realized that the short emarginate " spines *' were only 

 tough mucilaginous pi'ocesses which had been secreted through 

 pores in the cell-wall. Similar "spines*' or "processes" are 

 frequently met with in this genus (consult PI. CXIII, fig. 15, 

 and PL CXIV ; fig. 5 c) and they often become quite hard and 

 resistant, although they consist only of the more gelatinous 

 pectose constituents of the cell-wall. Mucous projections 

 (from the pores in the cell-wall) of a precisely similar nature 

 have been described and figured bv Liitkemiiller in A. 



*/ 



octocornis (consult Liitkem. ' Zellmembr. Desmid/ 1902, p. 412, 



t. 18, f. 17). 



Var. subsequalis car. nov. (PL CXVII, figs. 2, 3.) 



A subulatus as recorded by W. & G. S. West, Alg. N. Ireland, 1902, p. 5S ; 

 Scott. Freshw. Plankton, I, 1903, p. 528. 



Cells proportionately wider (without the spines) ; 

 semicells more narrowly elliptical with a less con- 



