80 BRITISH DESMIDIACFJE. 



ENGLAND. Cumberland ! Westmoreland ! (Bissett). 

 Lancashire ! W., N., and E. Yorks ! Gloucester, Sussex, 

 Kent, Hants, and Cornwall! (lialfs). 



WALES.- -Dolbadarn Castle, Llyn Padarn, Snowdon, 

 and Grlyder Fawr, Carnarvonshire ! 



SCOTLAND. --Sutherland, Aberdeen, Kincardine, For- 

 far, Perth, Dumbarton (Roy fy Bissett). Renfrew ! 



IRELAND. Lough Gruitane, Kerry ! Dublin and 

 Wicklow (Archer). Slieve Donard, Down ! 



Geogr. Distribution.- -France. Germany. Bohemia 

 and Galicia in Austria. Italy. Norway. Sweden. 

 S. Russia. Faeroes. United States. 



X. aculeatum is more widely distributed than X. JBrebissonii, 

 and is not an uncommon Desmid in the more mountainous 

 parts of the British Islands. The spines are arranged in a 

 continuous irregular hand around the greatest circumference 

 of the cell. The semicells vary somewhat in outline, and the 

 apex may he slightly elevated (as in Cosmarium ornatum) . 

 The central protuberance is generally much less prominent 

 than in X. Brebissonii, and also less granulated than in that 

 species, while immediately above it there is often situated a 

 spine or an emarginate wart. 



The forms with very short spines described as ' forma 

 brevispina' -have not yet been observed in the British 

 Islands. 



Var. basidentatum (Borg.) nob. (PI. CXVII, figs. 23, 



24.) 



X. Brebissonii var. l/asidentatum Borg. Bidrag- Bornli. Desm.-fl. 1889, 

 p. 148, t. 6, f. 11 ; Roy & Biss. Scott. Desm. 1893, p. 244; Schmidle, Alg. 

 Greb. Oberrheins, 1893, p. 552; Lutkem. Desm. Millstiittersees, 1900, 

 p. 14, t. 1, f . 15 ; Desm. Bohm. 1910, p. 496. 



Semicells a little more angular, with a more promi- 

 nently raised apex; spines irregular and much curved, 

 showing a tendency to grouping at the angles ; with 

 a group of 2 or 3 flattened verrucas just above but 

 close to the sinus on each side. 



Length without spines 60-78 /^, with spines 76- 

 93 ju, ; breadth without spines 52-67 /x, with spines 

 68-83 /*; breadth of isthmus 20-23 /x; thickness 

 42-48 p.. 



