ARTHRODESMUS. 115 



This curious form, unmistakable in its outline, we have 

 named forma SUCCISA W. & Gr. S. West ('New Brit. Freshw. 

 Alg.' 1894, p. 9, t. 2, f. 52) ; length 12'5/u; breadth ll'5 p . f 

 breadth of isthmus 3/u; thickness 6/i. (PI. CXVII, fig. 15). 



Var. latidivergens West. (PL CXVII, fig. 16.) 



A. bifidus var. latodivergens West, Alg. W. Ireland, 1892, p. 169, t. 22, 

 f. 8. 



Semicells with rather larger spines at the emarginate 



angles, the two spines at each lateral angle being 

 placed exactly at right angles. 



Length with spines 18 /x; breadth with spines 18 /x; 

 thickness 6'5 /x. 



IRELAND. Ballynahinch, Galway ! 



12. Arthrodesmus trispinatus W. & Gr. S. West. 



(PI. CXVII, fig. 17.) 



Arthrodesmus trispinatus W. & Gr. S. West, Alg. N. Ireland, 1902, p. 57, 

 t. 2, f . 16 ; Ckitw. Flor. Alg. Mont. Tatr. 1909, p. 466, t. 8, f . 40 [figures 

 very poor]. 



Cells minute, about as long as broad (without 

 spines), moderately constricted, sinus widely open and 

 rounded ; semicells transversely elliptic-oblong, with a 

 broadly concave apex, sides with three equal undula- 

 tions (three crests and two hollows) each furnished 

 with a short, stout spine, spines divergent. Vertical 

 view elliptic, showing the superposed spines at the 

 poles. Cell- wall smooth. 



Zygospore unknown. 



Length without spines ll'o/m, with spines 17/x,; 

 breadth without spines 11 //,, with spines 1 7 p. ; breadth 

 of isthmus 6*2 /x; thickness 7*3 /x ; length of spines 

 3 3'5 ju,. 



IRELAND.- -Lough Gar tan, Donegal ! 



Geogr. Distribution. Galicia in Austria. 



This plant is at once distinguished from all other species 

 of the genus by its small size and by the three spines on the 

 lateral margins of each semicell. It is perhaps nearest to 

 Arthrodesmus ineptus (Turn.) W. & Gr. S. West ( = Xanthi- 

 dium ineptum Turn. ' Freshw. Alg. E. India/ 1893, p. 101, 



