116 BRITISH 



t. 12, f. 21), an Indian species with irregularly disposed 

 spines and cells of a different shape. 



No living cells of this plant have yet been seen, and the 

 exact nature of the chloroplasts has not been determined. 

 We have suggested, therefore ('Alg. N. Ireland/ 1902, 

 p. 58), that it may be one of the Protococcales belonging to 

 the genus Tetraedron. If such were the case then "A. 

 trispinatus 3 would to some extent resemble a Tetraedron 

 which has been described on two separate occasions as one of 

 the Desmidiacese ; first by Wille (' Ferskv. Alg. Nov. SemljV 

 1879, p. 62, t. 13, f. 70) as " Sphserozosma excavatum Rail's 

 var. Novse Semliae Wille," and secondly by Schmidle (' Beitr. 

 alp. Alg/ 1895, p. 350, t. 15, f. 9) as " Xanthidium alpinum." 



13. Arthrodesmus tenuissimus Arch. 

 (PI. CXVII, figs. 18, 19.) 



Arthrodesmus tenuissimus Arch. Palmoglcea and descrip. Mesot. 1864, p. 68 



(sep. 28), t. 1, f. 50-55; Eabenh. Flor. Europ. Alg. Ill, 1868, p. 226; 



Cooke, Brit. Desni. 1887, p. 137, t. 47, f. 3 [figures inaccurate] ; Anderss. 



Sverig. Chlor. 1890, p. 13 ; Schmidle, Beitr. alp. Alg. 1896, p. 20 ; Nordst. 



Index Desmid. 1896, p. 251 ; W. & G-. S. West, Alg. N. Ireland, 1902, 



p. 59 ; Liitkem. Desm. Bohm. 1910, p. 497. 

 Xanthidium tenuissimum (Arch.) Turn. Freshw. Alg. E. India, 1893, p. 



137. 



Cells minute, a little broader than long (without 

 the minute spines), fairly deeply constricted, sinus 

 widely open, almost rectangular ; with a subacute 

 apex ; semicells subhexagonal, sides slightly concave 

 (almost straight), apex truncate, straight or slightly 

 concave, lateral angles each with a small, short, 

 horizontally-placed spine, apical angles eacli furnished 

 with a minute spine or mucro which is not easily seen 

 owing to its direction. Side view of semicell sub- 

 circular-depressed, with a minute divergent mucro on 

 each side of the apex. Vertical view broadly fusiform- 

 elliptic, poles acutely cuspidate, with a minute spine 

 on each side near the poles. Cell- wall smooth. 

 Chloroplasts axile, with a central pyrenoid in each 

 semicell. 



Zy go spore unknown. 



Length 8-10 //, ; breadth without spines 11-1 1*5 /x, 



