Scytonemacece 



323 



shorter cells, are characters which indicate a clear affinity with 

 the Scytonemacea3. 



J/. diploaiphon Gorn. var. Cumbn'ca'West is known from the English Lake 

 District ; thickness of internal sheath 6'5 9 /z. Species of this genus are 

 very rarely met with. 



Genus Scytonema Ag., 1824. [Petalonema Berkeley, 1833 ; 

 Schizosiphoii Kiitz., 1843 (in part); Symphyosiphon Kiitz., 1843 (in 

 part) ; Arthrosiphon Kiitz., 1845.] The filaments generally occur in 

 a dense intricate mass, and are at once characterized by their false 

 branches. These pseudo-branches arise in pairs between the 

 heterocysts, each pseudo-branch being a continuation of the 

 original trichome which has perforated its old sheath. The sheath 

 is tough, lamellose, and of a golden-yellow or brown colour ; it is 

 generally of even thickness, but in a few species (e.g. S. alatuni) in 

 which it is of great width, the margins are irregular. 



Fig. 148. A D, Scytonema mirnliile (Dillw.) Thur., from Cronkley Fell, N. Yorks. 

 ( x 440) ; C, apex of a ''branch'; D, organ of attachment at base of filament. 

 E, Tolypothrix l/ui/ita (Desv.) Wartm., from Riccall Common, E Yorks 

 (x440). 



.S'. M>/ochr<yiix Ag. and >$'. mirabile (Dillw.) Thur. [ = > s '. fi<jur<t.tuin Ag.] are 

 the two most abundant British species, the former often occurring on wet 

 rocks in large felt-like masses an inch in thickness ; thickness of fil. 18 36 p. ; 

 thickness of trichomes 6 12/*. S. alatuin (Berk.) Borzi is frequent on wet 

 rocks of carboniferous limestone, occurring as a brownish-green stratum 

 which is thickly encrusted with lime ; thickness of fil. 24 66 // ; thickness of 

 trichomes 9 15 p.. 



212 



