188 BRITISH DESMIDIACE.E. 



long (excluding the spines) ; deeply constricted, sinus 

 first nearly linear, then opening widely ; semicells sub- 

 trapeziform or subelliptical, apex truncate with 2 

 prominent apical emarginate spines or short bind pro- 

 cesses on the upper margin of each face ; lateral angles 

 very slightly produced and ending in two stout diverging 

 spines, which lie in the same vertical plane ; upper and 

 lower margins of angles with 2 verrucse or tiny spines ; 

 and with 2 emarginate spines or short spinous processes 

 projecting from each face. Vertical view triangular or 

 quadrangular, sides concave with 2 short processes, 

 emarginate at the apex, projecting from each, and with 

 2 others just within the margin ; angles very slightly 

 produced. 



Zygospore unknown. 



Length, without processes, 40-45 (JL ; with processes, 

 48-64 [ji ; breadth, without spines, 37-60 ^ ; with spines, 

 54-95 (ji ; breadth of isthmus 9-1 6 \JL . 



SCOTLAND. --Near Buchanty, and near Fowlis Wester, 

 Perth (Roy & Biss.). Rhiconich, Sutherland (N. C.). 

 Plankton of Loch Fadaghoda, Lewis, and at Harris, 

 Outer Hebrides ! 



Geogr. Distribution. Norway. Sweden. Finland. 

 Australia. LTnited States. 



This rare species is confined in the British Isles to the bogs of 

 the older palaeozoic and precambrian areas. The biradiate form 

 is known from S. Harris, Outer Hebrides (see PL CLIV, fig. 16). 



173. Staurastrum furcigerum Breb. 

 (PL CLVI, figs. 7, 8, 11.) 



Staurastrum furcigerum Breb. in Menegh. Syiiops. Desm. 1840, p. 22(3 ; 

 Arch, in Pritch. Infus. 1861, p. 743, t. 3, f. 32, 33 ; Rabenh. Flor. Europ. 

 Alg. 1868, p. 219 ; Arch, in Quart. Journ. Micr. Sci. 1871, p. 95 ; Xordst. 

 Norges Desm. 1873, p. 36 ; Kirchn. Alg. Schles. 1878, p. 167 ; Wolle, 

 Desm. U. S. 1884, p. 146, t. 48, f. 12, 13, t. 52, f. 23, 24 ; Cooke, Brit, 

 Desm. 1887, p. 178, t. 62, f. 1 ; Hansg. Prodr. Alg. Bohm. 1888, p. 215 ; 

 Nordst. Bornholm Desm. 1888, p. 207 ; De Toni, Syll. Alg. 1889, p. 1224 ; 

 Heimerl, Desm. alp. 1891, p. 607 ; West, Alg. W. Ireland, 1892, p. 186 ; 

 Alg. Engl. Lake Distr. 1892, p. 21 ; Roy & Biss. Scott, Desm. 1893, p. 20 ; 

 West & G. S. West, Alg. S. England, 1897, p. 496 ; G. S. West, Variation 

 Desm. 1899, p. 396 ; West &^G. S. West, Alg. X. Ireland, 1902, p. 47 ; 



