242 BRITISH DESMIDIACEJ]. 



Sutherland ! Ben Laoigh, Perth ! Harris and Lewis, 

 Outer Hebrides ! Orkneys ! Shetlands ! Rare in the 

 plankton. 



IRELAND.- -Donegal ! Mayo ! Galway ! Kerry ! 

 Down ! Antrim ! 



Geogr. Distribution. Gralicia in Austria. Sweden. 

 Bornholm. Faeroes. Iceland. Ecuador (var.). 



C. formosulum is a frequent and widely distributed species 

 in the British Islands, occurring in bogs and in marshy lakes, 

 and not infrequently in the plankton. It is allied to the large 

 forms of C. subcostatum, but need not be confused with them. 

 We find it a very distinctive species, and the margin has never 

 the incised character exhibited by C. subcostatum. The central 

 tumour is similar to that of C. costatum although the vertical 

 ridges are not so strongly granulate. 



C. mesochondrium Schmidle ( ( Alg. Denver, Colorado,' 

 1895, p. 85, fig. 3) appears to be but a reduced form of 

 C. formosulum Hoff. 



Var. Nathorstii (Boldt) West & G. S. West. 

 (PI. LXXXVIII, figs. 4, 5.) 



Cosmarium Nathorstii Boldt, Desm. Gronland, 1888, p. 20, t. 1, f. 21 ; De 

 Toni, Syll. Alg. 1889, p. 1021 ; Gutw. Flor. Glon. Okolic Lwowa, 1891, 

 p. 59 ; Norclst. Index Desm. 1896, p. 180 ; West & G-. S. West, Alg. S. 

 England, 1897, p. 490 ; Some Desm. U. S. 1898, p. 307 ; Borg. Freshw. 

 Alg. Fseroes, 1901, p. 227 ; Larsen, Freshw. Alg. E. Greenland, 1904, 

 p. 87. 



Ursinella Nathorstii Kuntze, Revis. gen. plant. 1891, p. 925. 



Cells proportionately a little broader ; semicells 

 with less rounded basal angles, lateral margins with 

 3-6 bigranulate (or emarginate) crenations, granules of 

 central tumour stronger and more pronounced. 



Length 45*6-53 ju, ; breadth 44*4-47 /x ; breadth of 

 isthmus 12*5-15 ju ; thickness 22*8-24^. 



ENGLAND. Sutton Park, Warwickshire ! Near 

 Chapel Wood, S.E. Surrey! 



Geogr. Distribution. - Gralicia in Austria (var.). 

 Faeroes. Greenland. United States. 



It does not seem possible to separate C. Nathorstii as a 

 species distinct from C. formosulum. There is a v r ery slight 

 difference in external form, there are rather more of the 



