288 IVesi : Phytoplankton of English Lake District. 



This characteristic plankton-variety of A. triangtdaris 

 occurred in several of the lakes, but not in any great profusion. 

 Long. 35-42 ji ; lat. sine spin. 25-32 jx, cum spin. 77-83//. 



A most interesting form with a triangular vertical view 

 occurred in the plankton of Easedale Tarn. This constitutes 

 one of the connecting forms between the genera Arthrodesmiis 

 and Staurastrum. It was observed intermingled with the more 

 usual elliptical variety, and might be called ' forma triqueira ' 

 (Fig. 5D). 



Fig. 5. A., Xanthidium subhastiferum West var. Murrayi W. & G. S. 

 West-, X 430; B., Arthrodesmus crassus W. & G. S. West, X 500; C, 

 A. triangularis Lagerh. var. subtriangularis (Borge) W. & G. S. West, 

 X 500; D., A. triangularis var. subtriangularis forma triquetra, X 500; 

 E., Staurastrum jaciiliferum West, X 500. 



9. Arthrodesmus crassus W. & G. S. West in ' Journ. 

 Linn. Soc.', XXXV., Nov. 1903, p. 541, t. 14, f. 8, 9. Stauras- 

 trum Sarsii Huitfeldt-Kaas, Planktonundersogelser, L, Norske 

 Vande, Christiania, 1906, pp. 55, 156, t. i, f. 11-17. 



This species occurred in great abundance in Ennerdale 

 Water. Long. 19.5-23 /x ; lat. (sine spin.) 19-21 /x ; lat. 

 isthm. 10 /A ; crass. 11.5-13 ju. (Fig. 5 b). It appears to be a 

 true plankton form in the British lake-areas. Huitfeldt-Kaas, 

 who found the same Desmid in the Norwegian lakes, made the 

 mistake of regarding the fibrillar structure of the enveloping 

 mucus as an armature of spines. 



10. Xanthidium subhastiferum West var. Murrayi 

 W. & G. S. West, 1. c. 1903, p. 540, t. 16, f. 6. 



Natura'ht, 



