44 BRITISH PESM1DIACE.E. 



isthmus 14-22 //,; breadth of apex 19-28 JJL; thickness 

 15-22 p.. 



ExciLAXD. Helvellyn,Westmoreland ! Baildoii Moor, 

 W. Yorks ! 



WALES.- -Near Capel Curig, Llyn-y-cwm-ffynon (at 

 1253 ft.), Snow-don (at 2000 ft.), and in a small ditch 

 by Llyn Ogwen, Carnarvonshire ! Dolgelly, Merioneth ! 



SCOTLAND. Vat of Culblean, Aberdeen ; Den of 

 G-arrol, Kincardine (Roy $ Bissett). Clova Tableland, 

 Forfar ! Skve in Inverness ! 



/ 



IRELAND.- -Achill Island, Mayo (at 1500 ft.) ! Carran- 

 tuohill, Kerry ! Slieve Donard, Down ! 



Geor/r. Distribution. Germany. Switzerland. Italy. 

 Sweden. N. Russia. Poland. Faeroes. Xova Zembla. 



This alpine and arctic Desmid is not infrequently met with 

 in the boggy springs and streams of the mountainous areas 

 of the British Isles. Its nearest allies are C. tatricum Eacib. 

 and C. anceps Lund., with both of which it should be care- 

 fully compared. The angularity and prominence of the 

 superior angles of the semicells constitute one of its most 

 distinctive features. 



AVe have figured a form of it from near Capel Curig, Car- 

 narvonshire, in which the semicells have rather more rect- 

 angular basal angles and a more deeply retuse apex (PI. 

 LXIX, fig. 9). 



C. decedens should also be compared with C, laticeps Grim. 

 (in Eabenh. ' Flor. Europ. Alg./ Ill, 1868, p. 168; Nordst. 

 ' Freshw. Alg. N. Zeal./ 1888, p. 57, t. 6, f. 10), a small species 

 to which it bears much resemblance. It may be that Grunow's 

 species is but a form of C. decedens, 



Var. sinuosum (Lund.) Eacib. (PI. LXIX, fig. 5.) 



Cosiitarit'.m quad rat urn Ealfs, var. in Ealfs' Brit. Desm. 1848, t. 15, f. 1 c. 

 C. sinuosum Lund. Desm. Suee. 1871, p. -17 ; Wolle, Desm. U.S. 1884, p. 65, 



t. 16, f. 2; Eoy & Biss. Scott, Desm. 1894, p. 174; Nordst. Index Desm. 



1896, p. 234. 



C. plicatum Eeinsch, B minus Eeiiisch, Alg-. Prom. Bon. Spei, 1877, p. 241. 

 C. plicatum Eeinsch var. sinuosum Cooke, Brit. Desm. 1886, p. 81, t. 36, f. 



4 ; West, Alg. N. Wales, 1890, p. 288. 

 Dysphinctium sinuosum (Lund.) Hansg. Prodr. Alg-eiifl. Bohm. 1888, p. 



244 ; De Toni, Syll. Alg. 1889, p. 894. 

 Gosmarium decedens a. sinuosum Eacib. Desm. Nowe, 1889, p. 88. 



This variety differs in the slightly deeper constric- 



