324 Myxopliycece 



Genus Tolypothrix Kiitz., 1843. [Indus. Hassallia Berkeley, 

 1845.] The filaments of this genus are very like those of Scytonema, 

 and the plants are primarily distinguished by the single pseudo- 

 branches. The latter are never geminate as in Scytonema and 

 they always issue in the region of the heterocysts. Both terrestrial 

 and aquatic species occur, and the sheaths, which are usually 

 thinner than those of Scytonema, may be either flexible or more or 

 less fragile. Hassallia could only be separated from Tolypothrix 

 by the ' fragility of its sheaths, and that character is totally in- 

 adequate as a generic distinction 1 . The heterocysts are sometimes 

 3-, 4-, or 5-seriate. 



T. lanata (Desv.) Wartm. (thickness of fil. 9 12'5 p. ; thickness of 

 trichomes 7'5 10 p. ; fig. 148 E) and T. tennis Kiitz. (thickness of fil. 8 10 p ; 

 thickness of trichomes 6 8 p) are the most frequent British species, occurring 

 amongst various aquatic plants in ponds and lakes. 



Genus Desmonema Berkeley & Thwaites, 1849. The fila- 

 ments of this genus exhibit a slight dichotomous branching, 

 and two or more trichomes are included in one sheath. The 

 heterocysts are only found at the base of the filaments. 



D. Wrangelii (Ag.) Born. & Flah. is a very rare British Alga, occurring in 

 penicillate tufts 5 6 mm. high ; thickness of trichomes 9 10 /*. 



Genus Diplocolon Nag., 1857. The filaments possess single 

 or geminate pseudo-branches of a similar nature to those of 

 Scytonema but much shorter and greatly contorted. They consist 

 of one trichome within a sheath, and a number of filaments and 

 pseudo-branches are included within a larger common sheath. 



D. Heppii Nag. is known from Yorkshire. It forms a brownish-green 

 gelatinous stratum on damp calcareous rocks ; thickness of fil. 20 28 p ; 

 thickness of trichomes 6 10 /a. 



Family 3. NOSTOCACEJE. 



The plants of the Nostocacese are of a simpler type than 

 any others of the Psilonematea? which possess heterocysts. The 

 filaments are never fixed by one extremity and they are never 

 branched. The trichomes consist of a single series of cells of 

 uniform character. The cells are frequently torulose and the 



1 West & G. S. West in Journ. Bot. July 1897, p. 266. 



