ST AURA STRUM. 147 



are similar to those in St. aversum, but in most cases are 

 rather stronger and firmer. Owing to the varying degrees 

 of convexity of the lower margin of the semicells the position 

 of the angular papillae is not quite constant, and they some- 

 times have a distinct upward direction (Consult PL CXXIII, 

 figs. 3 a and a'). 



The zy^'ospores recorded by Cushman (' Bull. Torr. Bot. 

 Club/ xxxii, 1905, p. 226, t. 8," f . 12, 13) under the name of 

 St. brevispinum do not appear to us to belong to this species, 

 but rather to some form of St. Dickiei. The angles of the 

 semicells as figured by Cushman are much too acutelv rounded 



O t/ t/ 



(exclusive of the short spines) for any form of St. brevi- 

 spinum. 



Liitkemiiller (' Desm. Millstattersees/ 1900, p. 78) has 

 described a " forma minima' of this species (length 2 

 breadth 22*5 /u ; breadth of isthmus 7*5 



Forma major W. & G. S. West. (PI. CXXIII, fig. 4.) 



St. brevispinum forma major W. & GK S. West, Phytoplaiikton Engl. Lake 

 Distr. 1909, p. 290, f . 6 E (p. 291). 



Cells larger and somewhat inflated. 



Length 59-63 ^ ; breadth 52-57 ^ ; breadth of 

 isthmus 16 17/x. 



ENGLAND. Plankton of Thirlmere, Cumberland ! 

 Plankton of Brothers' Water and Ullswater, West- 

 moreland ! 



Var. Boldtii Lagerh. 



St. brevispinum forma Boldt, Siber. Chlorophy. 1885, p. 113, t. 5, f. 30. 

 St. brevispinum var. Boldtii Lagerh. Chlor. Abessin. n. Kordofan, 1893,, 



p. 163. 

 St. brevispinum forma Boldtii Turn. Freshw. Alg. E. India, 1893, p. 130, 



t. 16, f. 45 (?). 



Cells proportionately longer, semicells more broadly 



elliptic. 



Length 47 /x ; breadth 35 /x; breadth of isthmus 



13-5 p. 



Geogr. Distribution. Siberia. Abyssinia. India (?). 



This variety as figured by Boldt does not occur in the 

 British Islands, but a form of it is known from Scotland : 



