116 BRITISH DESMIDIACE.E. 



4. Closterium porrectum Nordst. 

 (PI. XI, fig. 12.) 



Closterium porrectum Nordst. Desm. Brasil. 1870, p. 203, t. 2, f . 1 ; Alg. 



Brasil. 1877, p. 1(5 ; Boergesen, Desm. Brasil. 1890, p. 933. 

 Arthrodia porrecta Kuntze, Eevis. gen. plant. 1891, p. 884. 



Cells of medium size, about 12 times longer than 

 their diameter, very strongly arcuate, outer margin 

 about 1(30 of arc, inner margin not tumid, gradually 

 and uniformly attenuated to the apices, which are 

 narrow and obtusely rounded ; cell-wall pale yellow- 

 brown, strongly striate, about 6 striae visible across 

 the cell. 



Zygospore unknown. 



Distance between the apices 225-270 /A ; breadth 

 24^30 p. 



Unknown from the British Islands. 



Geoyr. Distribution. Brazil. 



Var. angustatum var. iiov. (PL XI, fig. 13.) 



Cells narrower than in the typical form and slightly 

 less curved, about 14 or 15 times longer than their 

 diameter, outer margin about 140-145 of arc ; cell- 

 wall brown in colour, 7 strong striae visible across the 

 cell. 



Distance between apices 195 ^u; breadth 15'5 /x. 



SCOTLAND. Near Balallan, Lewis, Outer Hebrides ! 



This variety stands intermediate between Cl. Archer lain tut 

 Cl. porrectum, and it bears a great resemblance to some of the 

 more sparsely striated forms of the former species. 



5. Closterium didymotocum Corda. 

 (PL XII, figs. 1-5.) * 



Closterium didymotocum Corda, in Aim. de Carlsbad. 1835, pp. 185, 190, 

 192, 209, t. 5, f. 64, 65 ; Ealfs, Brit. Desm. 1848, pp. 168-169, t. 28, 

 f. 7 a and b ; Arch, in Pritch. Infus. 1861, p. 746, t. 3, f. 39 ; Eabenh. 

 Flor. Europ. Algar. Ill, 1868, p. 125 ; Jacobs. Desm. Danem. 1875, p. 

 175, t. 7, f. 6 ; Delp. Desm. subalp. 1877, p. 103, t. 17, f. 31-37 ; Kirchn. 

 Al--. Srhlrs. 1878, p. 138 ; Wolle, Desm. U.S. 1884, p. 39, t. 8, f. 12, 13 ; 



