FRESHWATER ALGÆ FROM TUDDAL IN TELEMARK. 179 



194. Xanthidium cristatmn. Breb. 

 Rare. 



195. Xanthidium antilopœum. (Breb.) Ktitz. 

 Generally distributed, but never in quantities. 



196. Xanthidium aculeatum. Ehrbg. 

 forma triquetra. 



Very rare, only in a single sample. 



197. Xanthidium concinnum. Arch. 



Probably not very uncommon, but overlooked according to 

 its minute size. Size: long, 7,5 /n. Small form. 



Genus Arthrodesmus. Ehrbg. 1838. 



198. Arthrodesmus Incus. (Breb.) Hass. 

 Frequent, and often in abundance. 



198 a. forma perforata. Schmidle. 



This form occurred in a single sample. PI. V, Fig. 4. 



198 b. var. Ralfsii. W. & G. S. West. 



Not so abundant as the type. (From other parts of Nor- 

 way I have observed this variety to be much more abundant 

 than the type.) With zygospores. 



199. Arthrodesmus contr översus. W. & G. S. West. 

 Very rare. Size: long. 13,5;«, lat. 11,5 i^i, lat. isthm. 6 ^, 



crass. 6.5 ^t. 



200. Arthrodesmus octocornis. Ehrbg. 

 General, and sometimes in quantity. 



201. Arthrodesmus tenuissimus. Arch. 

 Very rare, only in a single sample. 



202. Arthrodesmus sp. 



Of this desmid I have only seen a single specimen, and I 

 do not venture to identity it with a known species, or to describe 

 it as new. It bears a striking resemblance to A. controversus 

 W. & G. S. West, but in many points it differs from this species. 

 The spines are much longer, and the apex is not so convex. 

 PI. Ill, Fig. 14. 



