26 BRITISH DESMIDIACE2E. 



Nordst. Freshw. Alg. N. Zeal. 1888, p. 78 ; De Toni, Syll. Alg. 1889, p. 



1018 ; Eacib. Desm. Nowe, 1889, p. 92 ; West, Alg. N. Wales, 1890, p. 



289 ; Heimerl, Desm. alp. 1891, p. 595 ; Roy & Biss. Scott, Desm. 1894, 



p. 43 ; Norclst. Index Desmid. 1896, p. 62 ; W. & G. S. West, Alg. S. 



England, 1897, p. 490 ; G. S. West, Variation Desm. 1899, p. 389, t. 10, 



f . 22-28 ; W. & G. S. West, Alga-fl. Yorks. 1900, p. 88 ; Freshw. Alg. 



Ceylon, 1902, p. 169 ; Larsen, Freshw. Alg. E. Greenland, 1904, p. 83. 

 C. quadrangulatum Hantzsch in Rabenh. Alg. Sachs. 1860, No. 969 ; 



Rabenh. Flor. Europ. Alg. Ill, 1868, p. 170 ; De Toni, Syll. Alg. 1889,. 



p. 1052. 



? C. anomalum Delp. Desm. siibalp. 1877, p. 29, t. 9, f. 14 [not f. 10-12, 15]. 

 C. biretum var. intermedium Wille, Ferskv. Alg. Nov. Semlj. 1879, p. 35, 



t. 12, f. 15. 

 C. biretum forma groenlandica Boldt, Desmid. Gronlaiid, 1888, p. 25, t. 2, 



f . 26. 

 C. biretum forma subconspersa Boldt, 1. c. p. 25 ; Nordst. in Wittr. & 



Nordst. Alg. Exsic. 1896, no. 1266 ; 1903, fasc. 35, p. 13. 

 Ursinella bireta Kuntze, Revis. gen. plant. 1891, p. 924. 

 U. quadranguldta Kuntze, 1. c. p. 925. 



Cells rather under medium size, about as long as 

 broad, very deeply constricted, sinus narrowly linear 

 witli a slightly dilated extremity ; semicells some- 

 what variable in outline, commonly subrectangular- 

 trapeziform, with the sides straight or slightly convex 

 and upwardly diverging, basal angles slightly rounded, 

 apical angles rounded (sometimes obliquely truncate or 

 even slightly produced), apex usually convex (some- 

 times truncate in the middle). Side view of semicell 

 subcircular or ovate. Vertical view elliptic, with a 

 protuberance of variable magnitude (sometimes scarcely 

 evident, at other times large) at the middle on each 

 side. Cell- wall densely granulate; granules small, 

 closely disposed in vertical series, or more frequently 

 more or less irregularly arranged, from 40 to 54 showing 

 at the margin of a semicell. Chloroplasts axile, with 

 two prominent pyrenoicls in each semicell. 



Zygospore globose and smooth. 



Length 54-74 //, ; breadth 50-60 p, ; breadth of 

 isthmus 19-24 //, ; thickness 35-42 p.. 



A smaller form, which gradually merges into the above, is 

 known as: 



Var. MINUS Hansg. Prodr. Algenfl. Bolnn. 1888, p. 251 ; 

 C. biretum forma minor Teodoresco, Mater, tlor. alg. Rouman. 

 1907, p. 181. 



Length 38-52 /u; breadth 35-52 /m- breadth of isthmus 13- 

 19 fjL-, thickness 23 /x; diam. zygosp. 39-45 /*. 



