ARTHEODESMUS. 117 



with spines 12'5-13'6/x; breadth of isthmus 5'9-6/x; 

 thickness 5-5'5 //,. 



ENGLAND.- -Helvellyn, Westmoreland ! 



WALES. Llyn Teyrn on Snowdon, Carnarvonshire ! 



SCOTLAND. - -Sutherland !, Ross, Aberdeen, Forfar !, 

 Perth (Itoij $- Bissett). Lewis and Harris, Outer 

 Hebrides ! 



IRELAND.- -Lough Fea, Londonderry ! Featherbed 

 Bog, Dublin Mountains (Archer). Ballynahinch, 

 Gralway ! 



Geo<jr. Distribution. France. Germany. Gralicia 

 and Bohemia in Austria. Arctic Norway. Sweden. 



This minute species is unlike any other in the genus. It 

 is very rare, hut when it does occur it is often in quantity 

 and generally amongst submerged Sphagnum. 



It is, perhaps, nearest to Xanthidium concinnum Arch., but 

 the semicells are of a somewhat different shape, the spines 

 on the lateral angles are much longer, and there is no central 

 papilla. The minute, paired apical spines do not occur in any 

 other species of Arthrodesmus. 



Forma longispina /. nov. (PI. CXVII, fig. 20.) 



A. tenuissimus Arch, as figured by West, Alg. N. Wales, 1890, t. 5, f. 10 

 [figure very poor and inaccurate as to detail] ; West, Alg. W. Ireland, 

 1902, p. 170, t. 22, f. 9. 



Lateral angles of semicells with much longer, 

 horizontally disposed spines. 



Length 8-12*5 /x,; breadth without spines 9 12 5 /x,; 

 with spines 27-3 1 /i ; lat. isthm. 4-5 ft; thickness 

 5*5-6 IJL. 



WALES. Capel Curig, Carnarvonshire ! 



IRELAND. Ballvnahinch and Louo'h Aunierin, 



/ o 



Gal way ! 



Under this form we call attention to the Desmid described 

 by Raciborski as Arthrodesmus liexagonus var. tumida (' Desm. 

 Tapakoomas./ 1895, p. 33, t. 4, f. 13) from British Guiana. 

 It appears to differ only in the central protuberance, and is 

 therefore one of those forms which connect the genera Arthro- 

 desmus and Staurastrum. 



