54 
IOWA ACADEMY OF SCIENCE 
This alga has been collected several times on the lower part of the stone 
wall on the arch in the college park. 
Ames. On stone wall. 1904. R. E. Buchanan. 
8. Chroococcus. 
11. C. re fr actus. 
Ames. 1884. C. E. Bessey. 
Order II. 
Hormogoneae. 
KEY TO THE SUBORDERS OF THE HORMOGONEAE. 
I. Trichomes cylindrical, sometimes narrowed at the extremities. 
Psilonemateae 
II. Trichomes conspicuously attenuated toward one or both extremeties, which 
are generally piliferous Trichophoreae 
Suborder I. 
Psilonemateae. 
KEY TO THE FAMILIES OF THE PSILONEMATEAE. 
I. Filaments with a false branch system; sheaths firm and tubular of more 
or less equal thickness; trichomes consisting of a single row of cells, 
with heterocysts, but not of uniform thickness Scytonemaceae 
II. Trichomes commonly tortuous and intricate, enveloped within a large 
gelatinous mass, consisting of a single row of uniform cells, generally 
torulose, with heterocysts; sheaths very delicate, mostly confluent 
/ Nostocaceae 
III. Trichomes consisting of a simple row of cells, uniform along their entire 
length except for the apical cells, which are sometime attenuated; 
heterocysts -absent; sheaths variable, more or less gelatinous, and some- 
times enclosing more than one trichome Oscillatoriaceae 
Family I, 
Scytonemaceae. 
1. Scytonema. 
KEY TO THE SPECIES. 
Superficially corrugated or hirsute, as if erose S. tomentosum 
Superficially smooth, sometimes the external layers dissolved into hairs 
S. myochrous 
12. S. tomentosum. 
Iowa City. 1880. Hobby. 
1905 Fink. 
IS. S. myochrous. 
Fayette. 1905. Fink. 
