308 Notes on the Blue-green Algae, etc. 
There are six well-marked families in the Hormogonez, 
(x) Oscillatoriaceee, (2) Nostocacez, (3) Scytonemaceze, (4) 
Stigonemacee, (5) Rivulariacee, (6) Camptotrichacee, and 
two in the Coccogonee. 
The Oscillatoriaceze contain a large number of species very 
widely distributed and frequently met with, belonging especially 
to the genera Oscillatoria, Phormidium, and Lyngbya. The 
primary distinctive characteristic of these three genera is the 
presence or absence of a gelatinous sheath. In Oscillatoria 
the sheath is absent; in the other two genera it is present, 
well-marked in Lyngbya, but usually not so well marked in 
Phornidium. Many species of this genus have a sheath which 
is so thin and transparent or diffluent that it is not easily made 
out without staining reagents, and it is extremely difficult, 
therefore, to differentiate them from species of Oscillatoria 
which they much resemble in other respects. In order to 
obviate this difficulty to some extent, and to allow of a more 
rapid preliminary diagnosis, I have for many years made use 
of a key for the species of these two genera, which is drawn up © 
in a purely artificial manner, and is based mainly upon the 
structure of the trichome as given in Gomont’s monograph*. 
The sheath is only used as a distinguishing feature when there 
can be no doubt of its visibility under the microscope. As I 
have found this key extremely useful, I have now made it 
more complete by the addition of references from De Toni’s 
“Sylloge Algarum ’+ and from Josephine Tilden’s monographt 
on the Myxophycee of North America, and venture to publish 
it, together with the more important characteristics of each 
species as given by Gomont and Tilden, in the hope that it 
may be useful to others. For a general account of the various 
families and genera of the Myxophycee the reader should 
consult Professor G. S. West’s treatise on British Freshwater 
Alge.§ References to some of the Yorkshire species will be 
found in ‘The Alge-Flora of Yorkshire,’ by W. and G. S. 
West, published by the Yorkshire Naturalists’ Union in 1r90T.|| 
(To be continued). 
* Gomont, ‘ Monographie des Oscillariees.. Ann. des Sci. Nat. Bot., 
1893. 
+ J. B. De Toni, ‘ Sylloge Algarum,’ Vol. 5 (‘ Sylloge Myxophycearum 
Omnium Hucusque Cognitarum,’ by Achilles Forti, 1907). 
t Josephine Tilden, ‘ Minnesota Alge’ (Vol. 1, ‘ The Myxophycez of 
North America and Adjacent Regions, including Central America, Green- 
land, Bermuda, the West Indies, and Hawaii.’ Report of the Survey, 
Botanical Series VIII., Minneapolis, 1910). 
§ G. S. West, ‘ A Treatise on the British Freshwater Alge.’ Cambridge 
University Press, 1904. 
|| Botanical Transactions of the Yorkshire Naturalists’ Union, Vol. 5. 
Leeds, I901. 
Naturalist, 
