162 BRITISH DESMIDIACE2E. 



sinus scarcely evident; semicells obtrapeziform, with 

 the sides slightly convex, apex truncate or sometimes 

 widely concave, apical angles obtuse and almost 

 imperceptibly produced. Side view of semicell semi- 

 elliptic. Vertical view elliptic, with the semicells not 

 in the same vertical plane. Cell- wall smooth. 



Zygospore unknown. 



Length 16-20/i; breadth 14-1 7 /x; breadth of 

 isthmus 10-11 /A; thickness 10*5 (JL. 



AY ALES.- -In boggy pools, Grlyder FacL, Carnar- 

 vonshire ! 



Geogr. Distribution.- -Sweden. 



We think this minute Desinid is better regarded as a 

 Staurastrum than, as a Cosmarium although it is obviously one 

 of those peculiar species which connect the two genera. A 

 marked twisting of the cell at the isthmus is a feature much 

 more often met with in Staurastrum than in Cosmarium. 



An Australian form of this species observed abundantly in 

 the plankton of the Yan Yean Reservoir, Victoria, was trigonal 

 in vertical view. The occurrence of this trigonal form (forma 

 trigona G. S. West in Mourn. Bot.' xliii, 1905, p. 253; < Alg. 

 Yan Yean/ 1909, p. 60, t. 5,, f . 3, 4) adds weight to the sug- 

 gestion that the species is best placed under the genus 

 Staurastrum. It has not been seen in the British Islands, 

 but we give a figure of it for comparison with the typical 

 elliptic form (PI. CXXV, fig. 10). 



25. Staurastrum subpygmseum West. 

 (PI. CXXV, figs. 13-16.) 



Staurastrum siibpygmseum West, Alg. W. Ireland, 1892, p. 178, t. 23, f. 8 ; 

 W. & GK S. West, New Brit. Freshw. Alg. 1894, p. 11, t. 2, f. 43 [forma 

 glabra] ; Alg. S. England, 1897, p 494 ; Further Contrib. Freshw. 

 Plankton Scott. Lochs, 1905, p. 486 ; Comp. Study Plankton Irish 

 Lakes, 1906, p. 86 ; Hrit. Freshw. Phytoplankton, etc., 19U9, p. 183. 



Cells of medium size, about as long as broad, deeply 

 constricted, sinus subrectang'ular with an acuminate 



o 



extremity ; semicells very widely cuneate, lateral 

 margins and apex convex, each lateral angle produced 

 into a hollow mamilla. Vertical view triangular, with 

 convex sides and mamillate angles. Cell-wall punctate 

 (or rarely smooth). 



