98 BRITISH DESMIDIACE2E. 



Cells rather small, a little longer than broad (without 

 the spines), deeply constricted, sinus a wide (often almost 

 semicircular) excavation, isthmus generally elongated 

 and shortly cylindrical ; semicells obversely triangular, 

 apex elevated and convex, retuse in the median part, 

 sides slightly convex, each lateral angle slightly 

 rounded and furnished with a long horizontally- 

 placed spine. Vertical view elliptic, each pole with 

 a long spine. One axile chloroplast in each semicell 

 containing a single pyrenoid. 



Zygospore unknown. 



Length (without spines) 23-30 //,; breadth without 

 spines 19-25 p.; length of spines 12*5-25 /x; breadth 

 of isthmus 5-7 JJL ; thickness 7*8-9 /x. 



ENGLAND.- -Plankton of Ennerclale Water and Crum- 

 mock Water, Cumberland ! Plankton of Grasmere, 

 Westmoreland ! 



WALES.- -Near Conway and Llyn Elsie, Carnarvon- 

 shire ! 



SCOTLAND. Plankton of Lochs Katrine and Achrav, 



i/ * 



Perth ! In the plankton of Loch na Cloiche Sgoilt, 

 Inverness ! Loch Beosetter, Bressay, Shetlands ! 



IRELAND.- -Near Glenties and near Lough Glentornan, 

 Donegal ! Ballynahinch, Gal way ! 



Geogr. DL*tribution.--(;xa.licm in Austria. Bengal. 

 Burma. United States. Brazil. Guiana. 



In arranging the British species of Arthrodesmus we have 

 considered it most advisable to retain A. triangularis and its 

 allied forms as a separate species-group. All these forms 

 constitute a very natural series, easily recognizable and 

 well demarcated from the numerous forms of A. Incus by the 

 form of the semicells and the horizontal disposition of the 

 long spines. Perhaps the most distinctive feature of A. 

 triangularis is the elevated apex of the semicells, a character 

 which is only possessed by one form of A. Incus, namely, A. 

 Incus var. Ralfsii forma siibJiexagona. 



We find this species much more abundant in the plankton 

 than in any other situations, and in many of these plankton- 

 forms the cells are often conspicuously twisted at the isthmus 

 (vide PI. CXIV, fig. 17). 



