CALOTRICHEA. 275 
Petronema fruticulosum. Thivaites. 
Plants densely cespitose, erect, somewhat regularly branched, 
branches free, with obtuse rounded apices, and each with a 
heterocyst at the base. Endochrome annulated, increasing in 
diameter towards the apices of the filaments. 
Sizz. Fronds ‘2 mm. long, trichomes 004 mm, diam. 
Petronema fruticulosum, Thwaites, Eng. Bot. ii, Supp., t. 
2959. 
Scytonema crustaceum, Ag. Syst., p. 89 (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 isin the Kew Herbarium (Herb. Hookerianum) a specimen of 
Scytonema crustaceum, Ag., received from that authority 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 Stigunema, 
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 CX11. fig. 2. Petronema fruticulosum after Thwaites; fig. 3, 
Scytonema crustaceum < 400, from authentic specimen received from 
Agardh. 
SuB-TRIBE Il. TRICHOPHOREZ. Filaments tapering at the top 
into a hyaline hair. 
Famity IV. CALOTRICHEA. 
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. 
