94 BRITISH DESMIDIACEJK. 



Length without spines 31 /x, with spines 45 ^ ; 

 breadth without spines 26 ju, with spines 56 /x, ; breadth 

 of isthmus 12'5//,. 



WALES.- -Llyn Idwal, Glaslyn, and Llyn-y-Ddinas, 

 Carnarvonshire ! 



Geogr. Distribution.- -Lappmark in Norway. 



The scrobic illations on the cell-wall of this form are doubt- 

 less situated each under the base of one of the mucilaginous 

 processes which have already been mentioned. 



Var. indentatus var. nov. (PL CXIII, figs. 20-24.) 



Sides of seinicells indented just above the isthmus, 

 making the semicell tranversely subrectangular in its 

 upper greater portion and somewhat cup-shaped in its 

 lower lesser portion ; spines long and stout, generally 

 divergent but more rarely horizontal. Cell-wall some- 



o / 



times irregularly and sparsely scrobiculate. 



Length without spines 26-31 p,; breadth without 

 spines 18-22/x; length of spines 17'5-26/i; breadth 

 of isthmus 7-9 p. 



ENGLAND.- -Cumberland ! Westmoreland ! N. Yorks ! 

 Cornwall ! 



WALES.- -Capel Curig, Llyn Ogwen, Llyn Idwal, 

 Llyn-y-cwm-ffynon, and Grlyder Fach, Carnarvonshire ! 

 Dolsrellv, Merioneth. 



O tj 



SCOTLAND. Sutherland! Perth! Forf ar ! Aberdeen! 

 Lewis and Harris, Outer Hebrides ! 



IRELAND.- -Donegal ! Down ! Mayo ! Galway ! Kerry ! 

 Cork ! 



This is one of the most frequent British varieties, occurring 

 principally at the boggy and weedy margins of lakes. It is 

 somewhat variable in the length and divergence of its spines, 

 but the slight constriction of the basal part of the semicell is 

 a very constant feature. Some of the cells are irregularly 

 scrobiculated, somewhat as in A. Incus forma perforata, and 

 these might be known as ' forma scrobiculata' (PL CXIIi, 

 fig. 24). 



It is possible that this variety is identical with A. Incites 

 forma isthmosa Heimerl (' Desm. alp.' 1891, p. 603, t. 5, f. 18) 



