240 BRITISH DESMIDIACE.E. 



most series and those within the basal angles, in the 

 centre above the isthmus with a large tumour furnished 

 with 5-7 vertical granulate ridges. Side view of semi- 



O O 



cell ovate or subrectangular, inflated near the base on 

 each side, upper angles rounded, apex convex or retuse. 

 Vertical view elliptic, with a broad crenulate inflation 

 on each side. Chloroplasts axile, with one pyrenoid. 



Zygospore unknown. 



Length 27-57*6 p.; breadth 25-39'6 p, ; breadth of 

 isthmus 10'5-21'G //,; thickness 15-28*8 p.. 



ENGLAND. --Chippenham Fen, Cambridgeshire ! 



WALES.- -Llyn. Idwal, Carnarvonshire ! 



SCOTLAND.- -Iron Hill, Banff; Aberdeen; in Mull, 

 Argyll (Roy $* Bissrft). Bressay, Shetlands ! 



Geogr. Distribution.- -Qalicia, in Austria. Sweden. 

 Xova Zembla. Franz Joseph Land. Greenland. 

 Spitzbergen. United States. 



This rare alpine and arctic Desmid exhibits considerable 

 variation in size. Boldt ('Desm. Gronland/ 1888, p. 21) 

 regards tlie smallest forms as forma minor and the largest 



O 



forms as forma major, 



Forma minor Boldt, Length 27-33'6 ju breadth 25-28*8 p ; 

 breadth of istlimns 10*5-12^; thickness 15-20*4 /<. 



Forma major Boldt. (= forma major Schmidle, c Lappmark 

 Siisswasseralg-en/ 1898, p. 38, t. 1, f. 58.) Length 45-57*6^; 

 breadth 36-39*6 ju; breadth of isthmus 18-21*6 /< ; thickness 

 26-4-28-8 fi. 



Nordstedt has described a triangular form from Spitz- 

 bergen (var. triquetrum ]STordst. 1875 [= C. abnorme var. 

 frlqnetrum Nordst. ' Desm. Spetsb/ 1872, p. 32, t. 6, f. J5]), 

 and Boldt lias also observed it from Greenland. 



The occurrence of C. costatum in the fens of the east of 

 England is one of those strange facts of distribution for which 

 as yet we can offer no explanation. 



206. Cosmarium formosulum Hoif. 

 (PL LXXXVIII, figs. 1-3.) 



? Cosmarium Quasittus Lund. var. quadrifera f. polycrenata Jacobs. Desm. 



Danem. 1876, p. 196, t. 7, f. 17 a. 

 C. formosulum Hoff in Xordst. Desm. Bornh. 1888, p. 194, t. 6, f. 6-7 ; De 



Toni, Syll. Alo-. 1889, p. 989 ; G-utw. Flor. Glon. Okolic Lwowa, 1891, 



