STAURASTRUM. 71 



or less oval they are subhexagono-trapezoid. The sinus is also 

 somewhat narrower. The real characters of St. Ravenelii and 

 St. Trelleckense are, however, still imperfectly known. 



92. Staurastrum erasum Breb. 

 (PL CXXXVII, figs. 9-11.) 



St. erasum Breb. Liste Desm. 1856, p. 143, t. 1, f. 28 ; Rabenh. Flor Eurm 

 Alg. 1868, p. 212 ; De Toni, Syll. Alg. 1889, p. 1147 ; Roy & Biss. Scott 

 Desm. 1893, p. 19: \\est & G. S. West, Notes Alg. IL 1900 p. 2!ii 



/""*1 ^ T\ 1 T^ ^ f\f\ "1 T /" A t f^ n 



Phytoplankt. 1909, p. 202 ; Gronblad, Desm. Keuru, 1920 p 6? t 

 f. 92-94. 



-<(. Brebissonii var. ordination Schmidle, Lappmark Siiss-vrasserak'en, 1898 

 p. 53, t. 3, f. 1. 



Cells small, about as long as broad, deeply constricted, 

 sinus acute, widening rapidly outwards ; semicells sub- 

 elliptical, ventral margin much more convex than the 

 dorsal, which is subtruncate ; angles broadly rounded, 

 and slightly retuse ; cell-wall thickly covered with 

 short spines arranged in concentric circles round the 

 anoles, where thev are longest, and becoming very much 



C? / v 



reduced towards the centre of the faces. Vertical view 

 triangular, sides concave, centre of apex nearly or quite 

 smooth. 



Zygospore unknown. 



Length, without spines, 30-42 [i ; breadth, without 

 spines, 30-42 ^ ; breadth of isthmus 10-11 [i; length 

 of spines at the angles, about 1'opi. 



ENGLAND.- -Plankton of Hayes Water, Westmore- 

 land ! Risley Bog, Lancashire (Roy). 



WALES. Glyder Fach (at 2200 feet), Carnarvonshire ! 

 In the plankton ! 



SCOTLAND. Near Coul, Ross (Roy & Biss.). Loch 

 Ruar, Sutherland ! L. Ghorma, Inverness ! Loch- 

 nagar, Aberdeen (up ' to 3500 feet) ! L. Doon, Ayr ! 

 Plankton of Lochs Brindister and Beosetter, Bressay, 

 Shet lands ! 



Geogr. Distribution .- -France. Germany, 

 (form). Finland. Siberia. United States (var.). 



