234 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
organ of protection. It is wanting in the Heterocystous Nostocaceae, 
and in the Homocystous genera protected by a thick sheath, as Schizothrix 
and some species of Lyngbya. Its structure is, however, often mani- 
fested only in the mature trichome before the formation of the hormo- 
gones. As a general rule the characters used for the discrimination of 
the genera are the number of tricbomes, whether one or more, in the 
sheath, the form and consistency of the latter, and the grouping of 
the filaments among one another ; while the anatomical characters of the 
trichome are chiefly employed for the distinction of species. The 
fourteen genera are thus classified : — 
Tribe I. Yaginarie^:. Trichomes two or more in each sheath 
when the filaments are completely developed (with one 
exception) ; sheath yellow, red, or blue. 
A. Sheath inclosing several trichomes. 1. Schizothrix (subgenera 
Inactis, Hypheotrix, Symphyosiphon, Chromosiphon). 2. 
Dasyglcea. 3. Microcoleus. 4. Hydrocoleum. 
B. Sheath red, inclosing only a single trichome. 5. Porphy- 
rosiphon . 
Tribe II. Lyngbye.®. Trichomes solitary in the sheath; sheath 
yellow, never red or blue. 
Snbtribe 1. Lyngbyoideae. Trichomes naked and mobile only 
for a short time ; hormogones secreting a new sheath when 
emerging from the one in which they were inclosed. 6. 
Plectonema. 7. Symploca. 8. Lyngbya (subgenera Leibleinia , 
Eulyngbya). 9. Phormidium. 
Subtribe 2. Oscillarioideas. Trichomes naked and mobile for 
the greater part of their existence ; sheath delicate, fragile, not 
coloured, in some species wanting or not yet detected. 10. 
Trichodesmium. 11. Oscillaria. 12. Borzia. 13. Arthrospira. 
14. Spirulina. 
Classification of Diatoms.* — Sig. H. Lanzi proposes the following 
classification of the Diatomaceae : — 
Series I. Frustula axi infravalvari saepius brevi, cingulo angusto 
et patente sculptura carente. 
A. Yalvae absque linea longitudinali mediana. 
a. Yalvis rotundatis. 
a. Sculptura simplice isomorpha (Melosireae, Coscino- 
disceas, Eupodiscete, Chaetocereae). 
b. Sculptura heteromorpha (Asterolampreae, Helio- 
pelteae). 
(3. Yalvis oblongis. 
a. Frustula cingulo et valvis asymmetricis (Meri- 
dioneae, Licmophoreae, Surirelleae). 
b. Frustula et cingulo symmetricis, valvis asym- 
metricis (Eunotieae). 
c. Frustula cingulo valvisque symmetricis (Nitzschieae, 
Fragilarieae, Tabellarieae). 
B. Yalvis linea et nodulo mediano praeditis. 
a. Frustula asymmetrica, cingulo recurvo dorsiventrali, 
* Atti Acc. Pont. Xuovi Lincei, xliii. ( 1890 ) pp. 53-7. Cf. this Journal, 1890, p. 496. 
