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shorter cells, are characters which indicate a clear affinity with 

 the Scytonemaceae. 



M. diplosiphon Gom. var. Cumbrica West is known from the English Lake 

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

 very rarely met with. 



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

 Schizosiphon Kiitz., 1843 (in part) ; Syrtiphyosiphon 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. alatum) in 

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



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

 ( x440) ; C, apex of a 'branch'; D, organ of attachment at base of filament. 

 E, Tolypothrix lanata (Desv.) Wartm., from Biccall Common, E. Yorks. 

 ( x 440). 



S. Myochrous Ag. and S. mirabile (Dillw.) Thur. [ = /<?. figuratum 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 p.. S. alatum (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 p, ; thickness of 

 trichomes 9 15 p.. 



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