CALOTRICHE^E. 275 



Petronema fruticulosuxn. Thwaites. 



Plants densely crespitose, erect, somewhat regularly branched, 

 branches free, with obtuse rounded apices, and each with a 

 heterocyst at the base. Endochroine annulated, increasing in 

 diameter towards the apices of the filaments. 



SIZE. Fronds "2 mm. long, trichomes '004 mm. diam. 



Petronema fniticulosum, Thwaites, Eng. Bot. ii., Supp., t. 

 2959. 



Scytonema cnistaceum, Ag. Syst., p. 39 (fide. spec, in Herb. 

 Hook.}. 



On rocks. Winter. 



" Forms a frustulose areolated olive brown crust upon the wet per- 

 pendicular surface of limestone rocks, to which it adheres so slightly as 

 to be very readily detached. The plants are densely crowded in little 

 hemispherical masses, which, together, produce the peculiar areolated 

 appearance of the crust. Each separate plant consists of a single fila- 

 ment at the base, which is from 1-4 or even more times tri- or tetra- 

 cholomously branched, and thus a furticose habit is given to the species. 

 Each branch has a connecting cell at its base, and sometimes one also at 

 about the middle of its length. The very thick cartilaginous sheaths are 

 of a pale brown colour towards the base of the plant but darker upwards, 

 except at the apices, which are frequently nearly colourless. The 

 endochrome is of a dull green colour, extremely narrow, and indistinctly 

 annulated ; towards the ends of the filaments, however, it becomes 

 much wider and slightly moniliform." 



There is in the Kew Herbarium (Herb. Hdokerianunt) a specimen of 

 Scytonema crustaceunt, Ag., received from that authoiity himself the 

 name written by him, which is clearly identical with Thwaites's plant, but 

 it could not consistently be united either with Scytonema or Stigonewa, 

 hence we have retained the name applied to it by Thwaites and placed 

 it here, subject to further investigation, its immediate allies being at 

 present doubtful. 



Plate CXI 1. Jiff. 2. Petronema fruticidosum after Thwaites; fig. 3, 

 Scytonema crustaceum X 400, from authentic specimen received from 

 Agardh. 



SUB-TRIBE ii. TRICHOPHOREJE. Filaments tapering at the top 



Into a hyaline hair. 



FAMILY IV. OALOTRIOHB^B. 



Filaments free, or agglutinated into a definite thallus, ter- 

 minating at the apex in a delicate hair-like extremity. Hetero- 

 cysts normally present, scattered, or basal. 



GENUS 113. CALOTHRIX. Ag. (1824.) 



Trichomes rather rigid, straight, attached, often fasciculate ; 

 growing in small tufts, or forming a turf of indefinite extent. 



