118 BRITISH DESMIDIACE^:. 



123. Staurastrum subgracillimum W. & G. S. West. 



(PL CXLIV, figs. 1, 2.) 



St. subgracillimum West & G. S. West, Some N. A. Desm. 1896, p. 263, t. 17, 

 f. 3, 4 ; Alg. N. Ireland, 1902, p. 56, t. 1, f. 21, 22 ; Alg. Ceylon, 1902, 

 p. 186 ; Further Contr. Freshw. Phytopl. Scott. Lochs, 1905, p. 504 ; 

 Brit, Freshw. Phytoplankton, 1909, p. 202. 



Cells small, about as long as broad, excluding the 

 processes ; semicells broadly cuneate, sides straight, apex 

 distinctly concave, upper angles produced to form very 

 long slender processes, which are nearly horizontal, and 

 of the same width throughout their whole length, with 

 margins minutely undulate. Vertical view triangular 

 or quadrangular, sides straight or very slightly concave, 

 angles produced to form long processes, the apices of 

 which are furnished with 3 spreading teeth ; processes 

 denticulate, cell-wall otherwise smooth. Cells often 

 twisted at the isthmus. 



Zygospore unknown. 



Length lG'5-15'5 VL ; breadth, without processes, 

 10-1 3 (ji ; with processes, 40-60 y* ; breadth of isthmus 

 4-8-6 (x. 



SCOTLAND. --Khiconich, Sutherland! Plankton of 

 small loch near Cearnabhal, Lewis, Outer Hebrides ! 



IRELAND .--Near Glenties, Co. Donegal ! 



Geogr. Distribution. Ceylon. United States. 



The distinctive features of this species are the concave apex, 

 and the long slender parallel processes. In the British Isles it is 

 a very rare Desmid, and is confined to bogs and lakes of the 

 ' western ' area.* The British examples all differ from the 

 original American specimens in their rather more robust processes, 

 and in their very minute apical spines. Both the American and 

 Ceylon specimens were provided with three larger spreading 

 teeth, all lying in the same horizontal plane. 



124. Staurastrum tetracerum Ralfs. 

 (PL CXLIX, figs. 2, 3.) 



? Micrasterias tetracera Kiitz. Syn. Diat. 1834, p. 74, t. 7, f. 83, 84. 

 Binatella tetracera Breb. Alg. Falaise, 1835, p. 269. 



* Seep. 18. 



