18 BRITISH DESMIDIACE.E. 



wanting in the plankton of the Midlands. Thus it is a truly 

 western type of Desmid, being confined to the drainage area of 

 the older palaeozoic rocks (see W. and G. S. W., ' Brit. Freshw. 

 Plankton/ 1909, p. 202). 



St. jaculiferum occurs in 2-4-radiate forms, the biradiate form 

 occurring in some plankton collections to the exclusion of all 

 other forms. The 3-radiate form is, however, the usual one, 

 and the 4-radiate form is comparatively rare. It has been proved 

 beyond all doubt that Arthrodesmus longicornis Roy & Biss. is 

 simply the biradiate form of this species, since specimens are 

 occasionally met in which one semicell is of the 2-radiate 

 form and the other 3-radiate. The biradiate form is also often 

 characterised by its slightly longer isthmus, and less divergent 

 spines. The inflexion of the spines seems, however, to be subject 

 to much variation in all forms of the species, the spines being 

 sometimes nearly parallel, and in other cases very divergent. 



The species is very well defined, and is nearest to St. aristi- 

 ferum Ralfs, from which it is very readily distinguished by the 

 more simple form of its semicells. 



Var. excavatum W. & G. S. West. (PL CXXXI, 



figs. 4, 5.) 



Staurastrum jaculiferum var. excavatum West & G. S. West, Scott. Freshw. 

 Plankton, I, 1903, p. 544, t. 17, f. 5. 



Differs from the type in its broadly obtuse sinus and 

 elongated cylindrical isthmus. 



Length, without spines, 26-27 JJL ; breadth, without 

 spines, 16-1 9 p. ; length 'of spines 21-29 pi ; breadth of 

 isthmus 8'5 JJL. 



SCOTLAND.- -Plankton of Loch Shin and Loch Ghriama, 

 Sutherland ! Plankton of Loch near Cearnabahl and 

 Loch Langabhat, Lewis, and Loch Diracleet, Harris, 

 Outer Hebrides ! 



IRELAND. Rare in plankton of Lough Caragh, Co. 

 Kerry ! 



Var. subexcavatum W. & G. S. West. (PI. CXXXI, 



fig. 6.) 



Staurastrum jaculiferum var. subexcavatum West & G. S. West, Scott 

 Freshw. Plankton, I, 1903, p. 544, t. 17, f. 6-8. 



