82 BRITISH DESMIDIACFJE. 



Zygospore globose, furnislied with numerous short 

 stout spines (or processes), trificl (rarely bifid) at the 

 apex. 



Length without spines 20-26 /* [-32 p. (Nordstedi)~] ; 

 breadth without spines 18-23 p. [-28 p, (Nordstedt)] ; 

 length of spines 2-3 /JL [4 ^ (Nordstedt)~\ ; breadth of 

 isthmus 6 8'6 JJL ; thickness 18'5-17 ^ ; diam. zygosp. 

 without spines 29-31 JJL, with spines 42-45 p. 



ENGLAND. Mickle Fell, N. Yorks ! Devils' Jumps, 

 Frensham (with zygospores), E slier Common, and 

 Thursley Common, Surrey ! 



IRELAND.- -Dungloe, near Glenties, near Gweedore, 

 Loup'h Machuo'h, and near Lone'li Glentornan, Donegal! 



o ~ ? o o 



Foxford and Clare Island, Mayo ! Near Oughterard 

 and Clifden, Galway ! Tore Mountain, Cromagloun, 

 and Carrantuohill, Kerry ! 



Geogr. Distribution.- -Ceylon. Australia. New Zea- 

 land. British Guiana. Patagonia. 



LJ 



We regard X. variabile as one of the most distinctive 

 species of the genus. It occurs in many parts of the British 

 Islands, more especially in certain peaty Sphagnum-bogs, in 

 which it is sometimes found in large numbers. It differs 

 from X. Smithii in its somewhat smaller size, its much more 

 protuberant central area, and in its much shorter and more 

 irregular spines. These points of difference are retained very 

 constantly, and not only does X. rar labile never occur 

 associated with X. Smithii, but no intermediate forms are 

 known which in any way tend to connect these species. In 

 fact we consider them to belong to different sections of the 

 gen u s Xa 1 1 1 hidium . 



X. variabile exhibits a considerable amount of variation 

 without losing any of its distinctive features. Each basal 

 angle possesses three short spines which show most distinctly 

 at the poles of the vertical view. The apical angles possess 

 either two or three spines. The central protuberance in the 

 great majority of specimens is in the form of a rather pro- 

 nounced papilla, but it may be truncate-emarginate or even 

 bispinate. 



The poles of the vertical view were described by Nordstedt 

 as truncate in the New Zealand specimens. In British 

 examples, however, they are generally rounded, or rarely 

 subtruncate. This is a point of little importance, although 



