220 BRITISH DESMIDIACE^:. 



of the cells of the filament has been observed in one 

 instance. 



The genus Spondylosium is very closely allied to Spkcerozosma, 

 the sole distinguishing feature being the absence of the short 

 apical processes between the cells which are characteristic of the 

 latter genus. 



All the British species are comparatively rare, and do not 

 commonly occur in abundance. They may be arranged as 

 follows : 



: Semicells elliptical or subelliptical. 



*} Apex of semicell straight or slightly convex, never 

 concave. 



1. Sp. pyymmun. 



2. Sp. ellipticum. 



3. Sp. planuin. 



4:. Sp. papillosum. 



5. Sp. Lundettii. 

 ft Apex of semicell distinctly concave. 



6. Sp. secedens. 



Cells more or less rectangular ; sinus a very shallow excava- 

 tion. 



7. Sp. tetragonum. 

 Semicells truncate-pyramidate. 



8. Sp. pulchellum. 



1. Spondylosium pygmaeum (Cooke) West. 

 (PL CLX, figs. 18, 19.) 



Sphcerozosma pygniceum Cooke, Brit. Desm. 1887, p. 5, t. 2. f. 5. [This is 

 not Sph. pygmceum Rabenh.] West, Alg. N. Wales, 1890, p. 6. 



Spondylosium pygmceum West, Freshw. Alg. W. Ireland, 1892, p. 11(3 

 (Sphcerozosma by printer's error) : West & G. S. West. Rec. publ. Desm. 

 1895, p. 65 ; Alga-fl. Yorks. 1902, p. 95. 



Cells minute, about as long as broad, sometimes 

 slightly broader than long, deeply constricted, sinus 

 acute, almost linear for part of its length ; semicells 

 elliptical, united by a relatively small surface of their 

 apices to form long filamentous colonies, frequently 

 enclosed in a gelatinous investment ; cell- wall smooth ; 

 vertical view subelliptic. 



Zygospore unknown. 



