340 



Myxophycece 



There are five British species, all of which are rare. They occur princi- 

 pally on dripping rocks in mountainous regions. D. Nordstedtii Born. 

 & Flah. and D. Orsiniana (Klitz.) Born. & Flah. (thickness of filaments 

 10 12 /u, of trichomes 6 7'5 p ; fig. 156 D) are widely distributed, and D. 

 interrupt i W. & G. S. West is known from the Mourne Mts, Down, Ireland 

 (fig. 156 C). 



Genus Rivularia (Roth) Ag., 1824 (in part); em. Thuret, 1885. 



[Zonatrichia J. Ag., 1842 ; 

 Limnactis Klitz., 1843 ; 

 Schizosiphon Kiitz., 1843 

 (in part).] The plants 

 consist of a globose or 

 hemispherical thallus, of a 

 tough character, composed 

 of radiating filaments which 

 are repeatedly ' branched.' 

 The thallus is attached to 

 submerged plants (such as 

 Chara, Myriophyllum, or 

 the bases of the stems of 

 Phragmites) or to the stones 

 of streams and cataracts, 

 and is sometimes indurated 

 with lime. The heterocysts 

 are basal and the extremi- 

 ties of the filaments are 

 piliferous. Spores have not 

 been observed. 



tt_-^e jgSjVj "** '"*'' 



There are four freshwater 

 species occurring in Britain, of 

 which R. hematites (D. C.) Ag. 

 [= Zonatrichia calcarea (Eng. 

 Bot.) Endlicher] is the most 

 frequent, sometimes occurring 

 in quantity attached to the 



stones of mountain streams in limestone districts. R. dura Roth and R. 



minulnlit (Kiitz.) Born. & Flah. (thickness of trichomes 2 12'5^; fig. 157 D 



and E) are occasionally met with attached to submerged plants, forming small 



globose masses of a blue-green or greenish-black colour. 



Genus Glceotrichia J. Ag., 1842. The thallus is globose and 

 free-floating, solid when young, but inflated and hollow when 



Fig. 1T>7. A C, Rivularia Biasolettiana 

 Menegh., from Aruclitfe, W. Yorks. ; A, nat. 

 size, on surface of stone; B and C, x 480. 

 D and E, R. mimHula (Kiitz.) Born. & Flah., 

 from Chippenham Fen, Cambridge; D, nat. 

 size, on stem of Phragmites; E, x 480. 



