134 BRITISH DESMIDIACE^. 



Phytoplankton, etc., 1909, p. 182; Htistedt, Desm. et Bacill. aus Tirol, 



1911, p. 337. 



St. muticum var. ellipticum Wolle, Desm. U. S. 1884, t. 39, f. 13. 

 St. muticum a. typicum Riabin. in Bull. Soc. Imp. Natur. Moscou, 1888, 



no. 2, p. 329. 

 St. depressum (Nag 1 .) Turn, forma aperta Turn. Freshw. Alg. E. India, 



1893, p. 104, t. 13, f. 19. 



Cells small, as long as broad or a little longer, deeply 

 constricted, sinus open and acute - angled ; semicells 

 usually elliptic, but often elliptic-semicircular or even 

 subrenif orm. Vertical view triangular or quadrangular, 

 with broadly rounded angles and concave sides. Cell- 

 wall smooth. 



Zygospore globose, furnished with stout spines which 

 are bifurcate at the apex \Ralfs~\. 



Length 22-43'5/ji; breadth 21-37'5/x; breadth of 

 isthmus 7'5-12/x; cliara. zygosp. without spines oS/x,; 

 length of spines 9 '5- 12 p.. 



ENGLAND.- -Westmoreland ! (Ralfs). W., N., and 

 E. Yorks! Essex! Warwick (Wills). Surrey! 

 Hants! Devon! (Bennett). Cornwall! 



WALES.- -Bethesda !, Snowdon !, and Capel Curig ! 

 (CooJce fy Wills), Carnarvonshire. Dolgelly, Merioneth 

 (Rolfs). Plankton of various Welsh lakes ! 



SCOTLAND.- -Sutherland !, Inverness, Aberdeen, Kin- 

 cardine, Perth, Stirling, Argyll (Roy $ Bissett). 

 Orkneys ! Shetlands ! 



IRELAND.- -Castlebar and Achill Island, Mayo! 

 Lough Guitane, Kerry ! Adrigole, Cork ! Dublin 

 and Wicklow (Archer). Plankton of Lough Neagh ! 



Geogr. Distribution. France. Germany. Austria 

 and Galicia. Hungary. Italy. Norway (and Fin- 

 mark). Sweden. Denmark. Bornholm. Finland. 

 N. and S. Russia. Faeroes. Spitzbergen. Green- 

 land (var.). Siberia. Mongolia. Java. Australia. 

 Central Africa. Azores (var.). Sandwich Islands. 

 United States. Brazil. 



St. muticum is a widely distributed species which sometimes 

 occurs in more or less abundance at the margins of pools and 

 lakes. The outward form of the semicells is somewhat vari- 

 able, and both triangular and quadrangular vertical views are 

 common. 



