178 BRITISH DESMIDIACE.E. 



This small species is one of the most characteristic and at 

 the same time one of the rarest of British Desmids. 



169. Cosmarium trachydermum West & G. S. West. 



(PI. LXXXII, fig. 1.) 



Cosmarium trachydermum West & G. S. West, Alg. Madag. 1895, p. 64, 

 t. 6, f . 26 ; Nordst. Index Desm. 1896, p. 256 ; Vest & G-. S. West, 

 Alga-fl. Yorks. 1900, p. 73. 



Cells rather under medium size, a little longer than 



o 



broad, deeply constricted, sinus somewhat open with 

 an obtuse extremity ; semicells elliptic-oblong, sides 

 rounded, apex truncate or subtruncate. Side yiew of 

 semicell subcircular. Vertical yiew elliptic, ratio of 

 axes about 1 : 1'7. Cell- wall eyenly granulate all oyer, 



e> O 



granules very small, without any definite arrangement, 

 29-30 visible at the margin. Chloroplasts axile, with 

 two pyrenoids. 



Zygospore unknown. 



Length 40*5^; breadth 06 n; breadth of isthmus 

 16 ju ; thickness 21 /LI. 



ENGLAND.- -Cowoill Wold Moss, Widdale Fell, W. 

 Yorks ! 



Geogr. Distribution.- -Madagascar. 



C. tr achy der mum has no near relatives among British species, 

 the rounded semicells and open sinus at once distinguishing' 

 it from all forms of C. punctulatum. 



170. Cosmarium sphseroideum West. 

 (PL LXXXII, figs. 2-4.) 



Cosmarium sphseroideum West, Alg. W. Ireland, 1892, p. 153, t. 21, f. 8 ; 

 Nordst. Index Desm. 1896, p. 237 ; West & G. S. West, Alg. N. Ireland, 

 1902, p. 37. 



Cells of medium size, about 1-| times as long as 

 broad, deeply constricted, sinus narrow and short, but 

 not completely closed ; semicells subcircular-oboyate, 

 with the widest part about two-thirds the distance 

 from the base, basal part truncately flattened, lower 

 parts of sides slightly convex and diverging upwards, 

 upper parts very broadly rounded, apex in the middle 

 truncate and not uncommonly very faintly retuse. 



