BRITISH DESMIDIE.K. 213 



XautkiJmm Artiscon differs from A', armatvvi by its segments tapering at 

 the base : its spines also are much longer and are more restricted to the outer 

 and rounded margin. 



Several plants referred by Ehrenberg to Xanthidium are angular in the end 

 view, and properly belong to Staurastrum. 



8. X. furcatum (Ehr.) ; " corpuscules globose, green, single orbinate, 

 spinous ; spines scattered, forked at the apex." 



Xanthidium furcatum, Ehrenberg, Infus. p. 148. t. 10. f. 25 (1838) ; 

 Meteor]), t. 1. f. 21. 



This plant is probably a Staurastrvm. M. de Brebisson refers it to S. 

 spivosum, and Professor Kiitzing makes it a synonym of his Thycastrvm fur- 

 cigerum (JDidymocladon furcigerus) . Of neither have I seen specimens re- 

 sembling Ehrenberg's figure, from which indeed I believe the latter plant is 

 very distinct. 



ARTHRODESMUS, Ehr. 



3. A. minutus (Kiitz.). 



Euastrum Incus, Kiitzing, Phycologia Germanica, p. 137 (1845) ; (not 



Staurastrum Incus, Meneghini). 

 Euastrum minutum, Kiitzing, in lit. cum icone (1846). 

 Arthrodesmus minutus, Brebisson, in lit. (1846). 



Germany, Kiitzing. 



Tab. XXXV. fig. 15. a, h. perfect fronds ; c. dividing frond : from drawings 

 by Professor Kiitzing. 



4. A. ? truncatus (Ehr.) ; " corpuscules green, slightly compressed, 

 campanulate, geminate, externally truncate, spinous/' 



Arthrodesmus truncatus, Ehrenberg, Infus. p. 152 (1838). 



This plant is probably either a Xanthidium or a Staurastrum. 



STAURASTRUM, Meyen. 



39. S. pygwKEum {Qr&h.) ', segments cuneiform; end view triangular, 

 with slightly rounded sides. 



Staurastrum pygmceum, Brebisson, in lit. cum icone (1846). 

 Falaise, Brebisson. 



Staurastrum pygmcBum is smaller than S. alternans, its sides, in the end 

 view, are more convex, and its angles less rounded. The sporangium is or- 

 bicular and spinous. 



Tab. XXXV. fig. 26. from drawings by M. de Brebisson: a. front view of 

 frond ; b. immature sporangium ; c. mature sporangium. 



p2 



